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SRUBAKER 


FOR    MEDICAL    STUDENTS. 


WILCOX.  MATERIA  MEDICA  AND  THERAPEUTICS:  IN- 
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LONG.  PHYSIOLOGICAL  CHEMISTRY.  By  John  Harper 
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BEARD.  TREATISE  ON  OPHTHALMIC  SURGERY.  By 
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cine in  Northwestern  University  Medical  School ;  Physician  to  St.  Luke's 
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O'REILLY.  A  MANUAL  OF  PHYSICAL  DIAGNOSIS.  By 
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THORINGTON.  REFRACTION  AND  HOW  TO  REFRACT. 
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HUMAN  PHYSIOLOGY. 

THIRTEENTH  EDITION. 


BRUBAKER. 


From  The  Southern  Clinic. 

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Wells,  m.  d.,  Associate  in  Obstetrics,  Jefferson  Medical  College,  Philadelphia, 

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ING.    Seventh  Revised  Edition. 
WELLS.     GYNECOLOGY.     Fourth  Edition.     With  153  Illustrations. 
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Treatment  and  Operations  and  a  Section  on  Local  Therapeutics.     With  Formulae 

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Text-book  of  Pharmacy. 
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ST.  CLAIR.     MEDICAL  LATIN.     Second  Edition. 
SCH AMBERG.     DISEASES  OF  THE  SKIN.     Fourth  Edition ,  Revised  and  Enlarged . 

108  Illustrations. 
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PITTFIELD.     BACTERIOLOGY.     With  4  Plates  and  80  other  Illustrations. 


BLAKISTON'S    ?  Q  U1Z-COMPENDS  ? 

A   COMPEND 

OF 

HUMAN    PHYSIOLOGY 

ESPECIALLY  ADAPTED  FOR  THE  USE 
OF  MEDICAL  STUDENTS 


BY 

ALBERT  P.  BRUBAKER,  A.  M.,  M.  D. 

AUTHOR  OF   "A  TEXT-BOOK   OF   PHYSIOLOGY";   PROFESSOR    OF    PHYSIOLOGY   AND 

MEDICAL  JURISPRUDENCE   IN  THE   JEFFERSON  MEDICAL  COLLEGE;   FORMERLY 

PROFESSOR  OF  PHYSIOLOGY   IN   THE   PENNSYLVANIA  COLLEGE   OF  DENTAL 

SURGERY;   LECTURER   ON   ANATOMY   AND  PHYSIOLOGY   IN  THE  DREXEL 

INSTITUTE   OF   ART,   SCIENCE,   AND    INDUSTRY;   FELLOW    OF  THE 

COLLEGE   OF   PHYSICIANS   OF   PHILADELPHIA 


THIRTEENTH  EDITION 
WITH  36  ILLUSTRATIONS 


PHILADELPHIA 

P.  BLAKISTON'S  SON  &  CO. 

1012   WALNUT   STREET 
1913 


%       JUL        194* 


Entered  according  to  Act  of  Congress,  in  the  year  19 12,    by 

P.  BLAKISfON'S  SON  &  CO. 

In  the  Office  of  the  Librarian  of  Congress,  at  Washington,  D.  C. 


Printed  by 

The  Maple  Press 

York,  Pa. 


a 


PREFACE  TO  THE  THIRTEENTH  EDITION. 


In  the  preparation  of  a  thirteenth  edition  of  the  Compend  an  opportunity 
has  been  furnished  for  the  revision  of  many  paragraphs  for  the  addition  of 
such  new  material  as  seemed  necessary  for  the  needs  of  students  during  their 
attendance  on  lectures  and  for  reviewing  the  subject  prior  to  their  examina- 
tions. That  it  may  continue  to  meet  the  needs  of  the  student  class  in  the 
future  as  it  has  in  the  past  is  the  sincere  wish  of  the  author. 

ALBERT  P.  BRUBAKER. 


Digitized  by  the  Internet  Archive 

in  2010  with  funding  from 
Columbia  University  Libraries 


http://www.archive.org/details/compendofhumanph1912brub 


CONTENTS. 


Page 
Introduction     i 

General  Structure  of  the  Animal  Body 3 

Chemic  Composition  of  the  Human  Body 7 

Physiology  of  the  Cell 25 

Histology  of  the  Epithelial  and  Connective  Tissues 31 

Physiology  of  the  Skeleton 38 

General  Physiology  of  Muscular  Tissue 42 

Special  Physiology  of  Muscles 54 

Physiology  of  Nerve  Tissue 60 

Foods  and  Dietetics 77 

Digestion 86 

Absorption 102 

Blood no 

Circulation  of  the  Blood     116 

Respiration 127 

Animal  Heat 135 

Secretion 137 

Mammary  Glands 140 

Vascular  Glands 142 

Excretion 148 

Kidneys     . 148 

Liver      155 

Skin 159 

The  Central  Organs  of  the  Nerve  System 162 

Spinal  Cord 164 

Medulla  Oblongata 174 

Pons  Varolii 178 

Crura  Cerebri 178 

Corpora  Quadrigemlna 179 

Corpora  Striata  and  Optic  Thalami 180 

Cerebellum .  181 

Cerebrum • 183 

Sympathetic  Nervous  System 193 

vii 


Vlll  CONTENTS. 

Cranial  Nerves 195 

Sense  of  Touch 209 

Sense  of  Taste 210 

Sense  of  Smell 212 

Sense  of  Sight 212 

Sense  of  Hearing     223 

Voice  and  Speech ' 231 

Reproduction 233 

Generative  Organs  of  the  Female 233 

Generative  Organs  of  the  Male 236 

Development  of  the  Embryo 238 

Table  Showing  Relation  of  Weights  and  Measures 243 

Index .    .    .   . 245 


A  COMPEND 

OF 


HUMAN  PHYSIOLOGY 


Introduction. — An  animal  organism  in  the  living  condition  exhibits  a 
series  of  phenomena  which  relate  to  growth,  movement,  mentality,  and  re- 
production. During  the  period  preceding  birth,  as  well  as  dining  the  period 
included  between  birth  and  adult  life,  the  individual  grows  in  size  and  com- 
plexity from  the  introduction  and  assimilation  of  material  from  without. 
Throughout  its  life  the  animal  exhibits  a  series  of  movements,  in  virtue  of 
which  it  not  only  changes  the  relation  of  one  part  of  its  body  to  another,  but 
also  changes  its  position  in  space.  If,  in  the  execution  of  these  movements,  the 
parts  are  directed  to  the  overcoming  of  opposing  forces,  such  as  gravity,  fric- 
tion, cohesion,  elasticity,  etc.,  the  animal  may  be  said  to  be  doing  work.  The 
result  of  normal  growth  is  the  attainment  of  a  physical  development  that  will 
enable  the  animal,  and,  more  especially,  man,  to  perform  the  work  necessitated 
by  the  nature  of  its  environment  and  the  character  of  its  organization.  In 
man,  and  probably  in  lower  animals  as  well,  mentality  manifests  itself  as  in- 
tellect, feeling,  and  volition.  At  a  definite  period  in  the  life  of  the  animal  it 
reproduces  itself,  in  consequence  of  which  the  species  to  which  it  belongs  is 
perpetuated. 

The  study  of  the  phenomena  of  growth,  movement,  mentality,  and  re- 
production constitutes  the  science  of  Animal  Physioiogy.  But  as  these 
general  activities  are  the  resultant  of  and  dependent  on  the  special  activities 
of  the  individual  structures  of  which  an  animal  body  is  composed,  Physiology 
in  its  more  restricted  and  generally  accepted  sense  is  the  science  which  in- 
vestigates the  actions  or  functions  of  the  individual  organs  and  tissues  of  the 
body  and  the  physical  and  chemic  conditions  which  underlie  and  determine 
them. 


2  HUMAN  PHYSIOLOGY. 

This  may  naturally  be  divided  into: 
i.  Special  physiology,  the  object  of  which  is  a  study  of  the  vital  phenomena 

or  functions  exhibited  by  the  organs  of  any  individual  animal. 
2.  Comparative  physiology,  the  object  of  which  is  a  comparison  of  the  vital 

phenomena  or  functions  exhibited  by  the  organs  of  two  or  more  animals, 

with  a  view  to  unfolding  their  points  of  resemblance  or  dissimilarity. 

Human  physiology  is  that  department  of  physiologic  science  which  has 
for  its  object  the  study  of  the  functions  of  the  organs  of  the  human  body  in  a 
state  of  health. 

Inasmuch  as  the  study  of  function,  or  physiology,  is  associated  with  and 
dependent  on  a  knowledge  of  structure,  or  anatomy,  it  is  essential  that  the 
student  should  have  a  general  acquaintance  not  only  with  the  structure  of 
man,  but  with  that  of  typical  forms  of  lower  animal  life  as  well. 

If  the  body  of  any  animal  be  dissected,  it  will  be  found  to  be  composed  of  a 
number  of  well-defined  structures,  such  as  heart,  lungs,  stomach,  brain,  eye,  etc., 
to  which  the  term  organ  was  originally  applied,  for  the  reason  that  they  were 
supposed  to  be  instruments  capable  of  performing  some  important  act  or  func- 
tion in  the  general  activities  of  the  body.  Though  the  term  organ  is  usually 
employed  to  designate  the  larger  and  more  familiar  structures  just  mentioned, 
it  is  equally  applicable  to  a  large  number  of  other  structures  which,  though 
possibly  less  obvious,  are  equally  important  in  maintaining  the  life  of  the  in- 
dividual— e.  g.,  bones,  muscles,  nerves,  skin,  teeth,  glands,  blood-vessels,  etc. 
Indeed,  any  complexly  organized  structure  capable  of  performing  some  func- 
tion may  be  described  as  an  organ.  A  description  of  the  various  organs 
which  make  up  the  body  of  an  animal,  their  external  form,  their  internal 
arrangement,  their  relations  to  one  another,  constitutes  the  science  of  Animal 
Anatomy. 

This  may  naturally  be  divided  into: 
i.  Special  anatomy,  the  object  of  which  is  the  investigation  of  the  construction, 

form,  and  arrangement  of  the  organs  of  any  individual  animal. 
2.  Comparative  anatomy,  the  object  of  which  is  a  comparison  of  the  organs  of 
two  or  more  animals,  with  a  view  to  determining  their  points  of  resem- 
blance or  dissimilarity. 

If  the  organs,  however,  are  subjected  to  a  further  analysis,  they  can  be 
resolved  into  simple  structures,  apparently  homogeneous,  to  which  the  name 
tissue  has  been  given — e.  g.,  epithelial,  connective,  muscle,  and  nerve  tissue. 
When  the  tissues  are  subjected  to  microscopic  analysis,  it  is  found  that  they 
are  not  homogeneous  in  structure,  but  composed  of  still  simpler  elements, 
termed  cells  and  fibers.     The  investigation  of  the  internal  structure  of  the 


GENERAL    STRUCTURE    OF   THE  ANIMAL   BODY.  3 

organs,  the  physical  properties  and  structure  of  the  tissues,  as  well  as  the 
structure  of  their  component  elements,  the  cells  and  fibers,  constitutes  a  de- 
partment of  anatomic  science  known  as  Histology,  or  as  it  is  prosecuted 
largely  with  the  microscope,  Microscopic  Anatomy. 

Human  anatomy  is  that  department  of  anatomic  science  which  has  for  its 
object  the  investigation  of  the  construction  of  the  human  body. 

GENERAL  STRUCTURE  OF  THE  ANIMAL  BODY. 

The  body  of  every  animal,  from  fish  to  man,  may  be  divided  into — 

1.  An  axial  and 

2.  An  appendicular  portion.  The  axial  portion  consists  of  the  head,  neck, 
and  trunk;  the  appendicular  portion  consists  of  the  anterior  and  posterior 
limbs  or  extremities. 

The  axial  portion  of  all  mammals,  to  which  class  man  zoologically  belongs, 
as  well  as  of  all  birds,  reptiles,  amphibians,  and  osseous  fish,  is  characterized 
by  the  presence  of  a  bony,  segmented  axis,  which  extends  in  a  longitudinal 
direction  from  before  backward,  and  which  is  known  as  the  vertebral  column 
or  backbone.  In  virtue  of  the  existence  of  this  column  all  the  class  of  animals 
just  mentioned  form  one  great  division  of  the  animal  kindom,  the  Vertebral  a. 

Each  segment,  or  vertebra,  of  this  axis  consists  of — 

1.  A  solid  portion,  known  as  the  body  or  centrum,  and 

2.  A  bony  arch  arising  from  the  dorsal  aspect  and  surmounted  by  a  spine- 
like process. 

At  the  anterior  extremity  of  the  body  of  the  animal  the  vertebrae  are  v  ari- 
ously  modified  and  expanded,  and,  with  the  addition  of  new  elements,  formthe 
skull;  at  the  posterior  extremity  they  rapidly  diminish  in  size,  and  terminate 
in  man  in  a  short,  tail-like  process.  In  many  animals,  however,  the  vertebral 
column  extends  for  a  considerable  distance  beyond  the  trunk  into  the  tail. 
The  vertebral  column  may  be  regarded  as  the  foundation  element  in  the  plan 
of  organization  of  all  the  higher  animals  and  the  center  around  which  the 
rest  of  the  body  is  developed  and  arranged  with  a  certain  degree  of  conformity. 
In  all  vertebrate  animals  the  bodies  of  the  segments  of  the  vertebral  column 
form  a  partition  which  serves  to  divide  the  trunk  of  the  body  into  two  cavities 
— viz.,  the  dorsal  and  the  ventral. 

The  dorsal  cavity  is  found  in  the  head.  Its  walls  are  not  only  in  the 
trunk,  but  also  formed  partly  by  the  arches  which  arise  from  the  posterior 
or  dorsal  surface  of  the  vertebrae  and  partly  by  the  bones  of  the  skull.  If 
a  longitudinal  section  be  made  through  the  center  of  the  vertebral  column, 
and  including  the  head,  the  dorsal  cavity  will  be  observed  running  through 
its  entire  extent.     (See  Fig.  1.)     Though  for  the  most  part  it  is  quite  narrow, 


HUMAN  PHYSIOLOGY. 


Fig.  i. — Diagrammatic  Longitudinal 
Section  op  the  Body. 

V,  V.  Bodies  of  the  vertebrae  which  divide 
the  body  into  the  dorsal  and  ventral  cav- 
ities, a,  a'.^  The  dorsal  cavity.  C,  p'. 
The  abdominal  and  thoracic  divisions  of 
the  ventral  cavity,  separated  from  each 
other  by  a  transverse  muscular  partition, 
the  diaphragm  d.  B.  The  brain.  Sp. 
C.  The  spinal  cord.  e.  The  esophagus. 
S.  The  stomach,  from  which  continues 
the  intestine  to  the  opening  of  the  pos- 
terior portion  of  the  body.  I.  The  liver. 
p.  The  pancreas,  k.  The  kidney,  o. 
The  bladder.  V .  The  lungs,  h.  The 
heart. 


at  the  anterior  extremity  it  is  en- 
larged and  forms  the  cavity  of  the 
skull.  This  cavity  is  lined  by  a 
membranous  canal,  the  neural 
canal,  in  which  is  contained  the 
brain  and  the  neural  or  spinal 
cord.  Through  openings  in  the 
sides  of  the  dorsal  cavity  nerves 
pass  out  which  connect  the  brain 
and  spinal  cord  with  all  the  struc- 
tures of  the  body. 

The  ventral  cavity  is  confined 
mainly  to  the  trunk  of  the  body. 
Its  walls  are  formed  by  muscles 
and  skin,  strengthened  in  most 
animals  by  bony  arches,  the  ribs. 
Within  the  ventral  cavity  is  con- 
tained a  musculo-membranous  tube 
or  canal  known  as  the  alimentary 
or  food  canal,  which  begins  at  the 
mouth  on  the  ventral  side  of  the 
head,  and,  after  passing  through 
the  neck  and  trunk,  terminates  at 
the  posterior  extremity  of  the  trunk 
at  the  anus.  It  may  be  divided 
into  mouth,  pharynx,  esophagus, 
stomach,  small  and  large  intestines. 

In  all  mammals  the  ventral 
cavity  is  divided  by  a  musculo- 
membranous  partition  into  two 
smaller  cavities,  the  thorax  and 
abdomen.  The  former  contains 
the  lungs,  heart  and  its  great 
blood-vessels,  and  the  anterior  part 
of  the  alimentary  canal,  the  gullet 
or  esophagus;  the  latter  contains 
the  continuation  of  the  alimentary 
canal — that  is,  the  stomach  and 
intestines — and  the  glands  in  con- 
nection with  it,  the  liver  and  pan- 


GENERAL    STRUCTURE    OF    THE  ANIMAL    BODY.  5 

creas.  In  the  posterior  portion  of  the  abdominal  cavity  are  found  the 
kidneys,  ureters,  and  bladder,  and  in  the  female  the  organs  of  reproduc- 
tion. The  thoracic  and  abdominal  cavities  are  each  lined  by  a  thin 
serous  membrane,  known,  respectively,  as  the  pleural  and  peritoneal 
membranes,  which,  in  addition,  are  reflected  over  the  surfaces  of  the 
organs  contained  within  them.  The  alimentary  canal  and  the  various 
cavities  connected  with  it  are  lined  throughout  by  a  mucous  membrane. 
The  surface  of  the  body  is  covered  by  the  skin.  This  is  composed  of  an 
inner  portion,  the  derma,  and  an  outer  portion,  the  epidermis.  The 
former  consists  of  fibers,  blood-vessels,  nerves,  etc.;  the  latter  of  layers 
of  scales  or  cells.  Embedded  within  the  skin  are  numbers  of  glands, 
which  exude,  in  the  different  classes  of  animals,  sweat,  oily  matter,  etc. 
Projecting  from  the  surface  of  the  skin  are  hairs,  bristles,  feathers,  claws. 
Beneath  the  skin  are  found  muscles,  bones,  blood-vessels,  nerves,  etc. 

The  appendicular  portion  of  the  body  consists  of  two  pairs  of  symmetric 
limbs,  which  project  from  the  sides  of  the  trunk,  and  which  bear  a  deter- 
minate relation  to  the  vertebral  column.  They  consist  fundamentally  of  bones 
surrounded  by  muscles,  blood-vessels,  nerves  and  lymphatics.  The  limbs, 
though  having  a  common  plan  of  organization,  are  modified  in  form  and 
adapted  for  prehension  and  locomotion  in  accordance  with  the  needs  of  the 
animal. 

Anatomic  Systems. — All  the  organs  of  the  body  which  have  certain 
peculiarities  of  structure  in  common  are  classified  by  anatomists  into  systems 
— e.  g.,  the  bones,  collectively,  constitute  the  bony  or  osseous  system;  the 
muscles,  the  nerves,  the  skin,  constitute,  respectively,  the  muscular,  the  ner- 
vous, and  the  tegumentary  system. 

Physiologic  Apparatus. — More  important  from  a  physiologic  point  of 
view  than  a  classification  of  organs  based  on  similarities  of  structure  is  the 
natural  association  of  two  or  more  organs  acting  together  for  the  accomplish- 
ment of  some  definite  object,  and  to  which  the  term  physiologic  apparatus 
has  been  applied.  While  in  the  community  of  organs  which  together  con- 
stitute the  animal  body  each  one  performs  some  definite  function,  and  the 
harmonious  cooperation  of  all  is  necessary  to  the  fife  of  the  individual,  every- 
where it  is  found  that  two  or  more  organs  though  performing  totally  distinct 
functions,  are  cooperating  for  the  accomplishment  of  some  larger  or  com- 
pound function  in  which  their  individual  functions  are  blended — e.  g.,  the 
mouth,  stomach,  and  intestines,  with  the  glands  connected  with  them, 
constitute  the  digestive  apparatus,  the  object  or  function  of  which  is  the 
complete  digestion  of  the  food.     The  capillary  blood-vessels  and  lymphatic 


6  HUMAN  PHYSIOLOGY. 

vessels  of  the  body,  and  especially  those  in  relation  to  the  villi  of  the  small 
intestine,  constitute  the  absorptive  apparatus,  the  function  of  which  is  the  in- 
troduction of  new  material  into  the  blood.  The  heart  and  blood-vessels 
constitute  the  circulatory  apparatus,  the  function  of  which  is  the  distribution 
of  blood  to  all  portions  of  the  body.  The  lungs  and  trachea,  together  with 
the  diaphragm  and  the  walls  of  the  chest,  constitute  the  respiratory  apparatus, 
the  function  of  which  is  the  introduction  of  oxygen  into  the  blood  and  the 
elimination  from  it  of  carbon  dioxid  and  other  injurious  products.  The 
kidneys,  the  ureters,  and  the  bladder  constitute  the  urinary  apparatus.  The 
skin,  with  its  sweat-glands,  constitutes  the  perspiratory  apparatus,  the  func- 
tions of  both  being  the  excretion  of  waste  products  from  the  body.  The  liver, 
the  pancreas,  the  mammary  glands,  as  well  as  other  glands,  each  form  a 
secretory  apparatus  which  elaborates  some  specific  material  necessary  to  the 
nutrition  of  the  individual.  The  functions  of  these  different  physiologic 
apparatus — e.  g.,  digestion,  absorption  of  food,  elaboration  of  blood,  circu- 
lation of  blood,  respiration,  production  of  heat,  secretion,  and  excretion — 
are  classified  as  nutritive  functions,  and  have  for  their  final  object  the  preserva- 
tion of  the  individual. 

The  nerves  and  muscles  constitute  the  nervo-muscular  apparatus,  the 
function  of  which  is  the  production  of  motion.  The  eye,  the  ear,  the  nose, 
the  tongue,  and  the  skin,  with  their  related  structures,  constitute,  respectively, 
the  visual,  auditory,  olfactory,  gustatory,  and  tactile  apparatus,  the  function 
of  which,  as  a  whole,  is  the  reception  of  impressions  and  the  transmission 
of  nerve  impulses  to  the  brain,  where  they  give  rise  to  visual,  auditory, 
olfactory,  gustatory,  and  tactile  sensations. 

The  brain,  in  association  with  the  sense  organs,  forms  an  apparatus  related 
to  mental  processes.  The  larynx  and  its  accessory  organs — the  lungs, 
trachea,  respiratory  muscles,  the  mouth  and  resonant  cavities  of  the  face — 
form  the  vocal  and  articulating  apparatus,  by  means  of  which  voice  and 
articulate  speech  are  produced.  The  functions  exhibited  by  the  apparatus 
just  mentioned — viz.,  motion,  sensation,  language,  mental  and  moral  mani- 
festations— are  classified  as  functions  of  relation,  as  they  serve  to  bring  the 
individual  into  conscious  relationship  with  the  external  world. 

The  ovaries  and  the  testes  are  the  essential  reproductive  organs,  the  former 
producing  the  germ-cell,  the  latter  the  spermatic  element;  together  with  their 
related  structures — the  fallopian  tubes,  uterus,  and  vagina  in  the  female, 
and  the  urogenital  canal  in  the  male — they  constitute  the  reproductive 
apparatus  characteristic  of  the  two  sexes.  Their  cooperation  results  in  the 
union  of  the  germ-cell  and  spermatic  element  and  the  consequent  develop- 
ment of  a  new  being.  The  function  of  reproduction  serves  to  perpetuate  the 
species  to  which  the  individual  belongs. 


CHEMIC    COMPOSITION    OF    THE   HUMAN    BODY.  J 

The  animal  body  is  therefore  not  a  homogeneous  organism,  but  one 
composed  of  a  large  number  of  widely  dissimilar  but  related  organs.  But 
as  all  vertebrate  animals  have  the  same  general  plan  of  organization,  there 
is  a  marked  similarity  both  in  form  and  structure  among  corresponding  parts 
of  different  animals.  Hence  it  is  that  in  the  study  of  human  anatomy  a 
knowledge  of  the  form,  construction,  and  arrangement  of  the  organs  in 
different  types  of  animal  life  is  essential  to  its  correct  interpretation;  also  it  is 
that  in  the  investigation  and  comprehension  of  the  complex  problems  of 
human  physiology  a  knowledge  of  the  functions  of  the  organs  as  they  manifest 
themselves  in  the  different  types  of  animal  life  is  indispensable.  As  many 
of  the  functions  of  the  human  body  are  not  only  complex,  but  the  organs 
exhibiting  them  are  practically  inaccessible  to  investigation,  we  must  sup- 
plement our  knowledge  and  judge  of  their  functions  by  analogy,  by  attributing 
to  them,  within  certain  limits,  the  functions  revealed  by  experimentation 
upon  the  corresponding  but  simpler  organs  of  lower  animals.  This  ex 
perimental  knowledge  corrected  by  a  study  of  the  clinical  phenomena  of 
disease  and  the  results  of  post-mortem  investigations,  forms  the  basis  of 
modern  human  physiology. 


CHEMIC  COMPOSITION  OF  THE  HUMAN  BODY. 

Since  it  has  been  demonstrated  that  every  exhibition  of  functional  activity 
is  associated  with  changes  of  structure,  it  has  been  apparent  that  a  knowl- 
edge of  the  chemic  composition  of  the  body,  not  only  when  in  a  state  of  rest, 
but  to  a  far  greater  degree  when  in  a  state  of  activity,  is  necessary  to  a 
correct  understanding  of  the  intimate  nature  of  physiologic  processes. 
Though  the  analysis  of  the  dead  body  is  comparatively  easy,  the  determina- 
tion of  the  successive  changes  in  composition  of  the  living  body  is  attended 
with  many  difficulties.  The  living  material,  the  bioplasm,  is  not  only  com- 
plex and  unstable  in  composition,  but  extremely  sensitive  to  all  physical  and 
chemic  influences.  The  methods,  therefore,  which  are  employed  for  analysis 
destroy  its  composition  and  vitality,  and  the  products  which  are  obtained 
are  peculiar  to  dead  rather  than  to  living  material. 

Chemic  analysis,  therefore,  may  be  directed — 
i.  To  the  determination  of  the  composition  of  the  dead  body. 
2.  To   the   determination   of  the  successive  changes  in  composition  which 

the  living  bioplasm  undergoes  during  functional  activity. 

A  chemic  analysis  of  the  dead  body,  with  a  view  to  disclosing  the  substances 
of  which  it  is  composed,  their  properties,  their  intimate  structure,  their 
relationship  to  one  another,   constitutes  what  might  be  termed   Chemic 


8  HUMAN  PHYSIOLOGY. 

Anatomy.  An  investigation  of  the  living  material  and  of  the  successive 
changes  it  undergoes  in  the  performance  of  its  functions  constitutes  what  has 
been  termed  Chemic  Physiology  or  Pbysiologic  Chemistry. 

By  chemic  analysis  the  animal  body  can  be  reduced  to  a  number  of  liquid 
and  solid  compounds  which  belong  to  both  the  inorganic  and  organic  worlds. 
These  compounds,  resulting  from  a  proximate  analysis,  have  been  termed 
proximate  principles.  That  they  may  merit  this  term,  however,  they  must 
be  obtained  in  the  form  under  which  they  exist  in  the  living  condition.  The 
organic  compounds  consist  of  representatives  of  the  carbohydrate,  fatty,  and 
proteid  groups  of  organic  bodies;  the  inorganic  compounds  consist  of  water, 
various  acids,  and  inorganic  salts. 

The  compounds  or  proximate  principles  thus  obtained  can  be  further 
resolved  by  an  ultimate  analysis  into  a  small  number  of  chemic  elements 
which  are  identical  with  elements  found  in  many  other  organic  as  well  as 
inorganic  compounds.  The  different  chemic  elements  which  are  thus  ob- 
tained, and  the  percentage  in  which  they  exist  in  the  body,  are  as  follows — 
viz.,  oxygen,  72  per  cent.;  hydrogen,  9.10;  nitrogen,  2.5;  carbon,  13.50; 
phosphorus,  1.15;  calcium,  1.30;  sulphur,  0.147;  sodium,  0.10;  potassium, 
0.026;  chlorin,  0.085;  fluorin,  iron,  silicon,  magnesium,  in  small  and  vari- 
able amounts. 

THE  CARBOHYDRATES. 

The  carbohydrates  constitute  a  group  of  organic  bodies,  consisting  mainly 
of  starches  and  sugars,  having  their  origin  for  the  most  part  in  the  vegetable 
world.  In  many  respects  they  are  closely  related,  and  by  appropriate  means 
are  readily  converted  into  one  another.  In  composition  they  consist  of  the 
elements  carbon,  hydrogen,  and  oxygen.  As  their  name  implies,  the  hydro- 
gen and  oxygen  are  present  in  the  majority  of  these  compounds  in  the  pro- 
portion to  form  water,  or  as  2:1.  The  molecule  of  the  carbohydrates  just 
mentioned  consists  of  either  six  atoms  of  carbon  or  a  multiple  of  six;  in  the 
latter  case  the  quantity  of  hydrogen  and  oxygen  taken  up  by  the  carbon  is 
increased,  though  the  ratio  remains  unchanged. 

The  carbohydrates  may  be  divided  into  three  groups — viz:  (1)  amyloses, 
including  starch,  dextrin,  glycogen,  and  cellulose;  (2)  dextroses,  including 
dextrose,  levulose,  galactose;  (3)  saccharoses,  including  saccharose,  lactose, 
and  maltose.  According  to  the  number  of  carbon  atoms  entering  into 
the  second  group  (six),  they  are  frequently  termed  monosaccharids;  those 
of  the  third  group,  disaccharids — twice  six;  those  of  the  first  group,  polysac- 
charids — multiples  of  six. 

Though  but  few  of  the  members  of  the  carbohydrate  group  are  constituents 


CHEMIC    COMPOSITION    OF    THE   HUMAN    BODY.  9 

of  the  human  body,  yet  on  account  of  their  importance  as  foods,  and  their 
relation  to  one  another,  a  few  of  their  chemic  features  will  be  stated  in  this 
connection. 

i.  AMYLOSES  (C6H10O5)n. 

Starch  is  widely  distributed  in  the  vegetable  world,  being  abundant  in  the 
seeds  of  the  cereals,  leguminous  plants,  and  in  the  tubers  and  roots  of  some 
vegetables.  It  occurs  in  the  form  of  microscopic  granules,  which  vary  in 
size,  shape,  and  appearance,  according  to  the  plant  from  which  they  are  ob- 
tained. Each  granule  presents  a  nucleus,  or  hilum,  around  which  is  arranged 
a  series  of  eccentric  rings,  alternately  light  and  dark.  The  granule  consists 
of  an  envelope  and  stroma  of  cellulose,  containing  in  its  meshes  the  true  starch 
material — granulose.  Starch  is  insoluble  in  cold  water  and  alcohol.  When 
heated  with  water  up  to  700  C,  the  granules  swell,  rupture,  and  liberate  the 
granulose,  which  forms  an  apparent  solution;  if  present  in  sufficient  quan- 
tity, it  forms  a  gelatinous  mass  termed  starch  paste.  On  the  addition  of 
iodin,  starch  strikes  a  characteristic  deep  blue  color;  the  compound  formed 
— iodid  of  starch — is  weak,  and  the  color  disappears  on  heating,  but  reappears 
on  cooling. 

Boiling  starch  with  dilute  sulphuric  acid  (twenty-five  per  cent.)  converts 
it  into  dextrose.  In  the  presence  of  vegetable  diastase  or  animal  ferments, 
starch  is  converted  into  maltose  and  dextrose,  two  forms  of  sugar. 

Dextrin  is  a  substance  formed  as  an  intermediate  product  in  the  transfor- 
mation of  starch  into  sugar.  There  are  at  least  two  principal  varieties — 
erythrodextrin,  which  strikes  a  red  color  with  iodin,  and  achr 00 dextrin,  which 
is  without  color  when  treated  with  this  reagent.  In  the  pure  state  dextrin 
is  a  yellow-white  powder,  soluble  in  water,  in  the  presence  of  animal 
ferments  erythrodextrin  is  converted  into  maltrose. 

Glycogen  is  a  constituent  of  the  animal  liver,  and,  to  a  slight  extent,  of 
muscles  0.5  to  0.9  per  cent,  and  of  tissues  generally.  In  the  tissues  of  the 
embryo  it  is  especially  abundant.  When  obtained  in  a  pure  state  it  is  an 
amorphous,  white  powder.  It  is  soluble  in  water,  forming  an  opalescent 
solution.  With  iodin  it  strikes  a  port- wine  color.  In  some  respects  it  resem- 
bles starch,  in  others  dextrin.  Like  vegetable  starch,  glycogen  or  animal 
starch  can  be  converted  by  dilute  acids  and  ferments  into  sugar  (maltose). 

Cellulose  is  the  basis  material  of  the  more  or  less  solid  framework  of  plants. 
It  is  soluble  only  in  an  ammoniacal  solution  of  cupric  oxid,  from  which  it  can 
be  precipitated  by  acids.  It  is  an  amorphous  powder;  dilute  acids  can  con- 
vert it  into  dextrose. 


10  HUMAN  PHYSIOLOGY. 

2.  DEXTROSES,  C6H1206. 

Dextrose,  glucose,  or  grape-sugar  is  found  in  grapes,  most  sweet  fruits, 
and  honey,  and  as  a  normal  constituent  of  liver,  blood,  muscles,  and  other 
animal  tissues.     In  the  disease  diabetes  mellitus  it  is  found  also  in  the  urine. 

When  obtained  from  any  source,  it  is  solule  in  water  and  in  hot  alcohol,  from 
which  it  crystalizes  in  six-sided  tables  or  prisms.  As  usually  met  with  it  is  in 
the  form  of  irregular,  warty  masses.  It  is  sweet  to  the  taste;  less  so,  however, 
than  cane  sugar.  It  is  dextro-rotary,  turning  the  plane  of  polarized  light  to 
the  right.  In  alkaline  solutions  dextrose  absorbs  oxygen,  and  hence  in  the 
presence  of  metallic  salts,  copper,  bismuth,  silver,  etc.,  it  acts  as  a  reducing 
agent.  On  this  property  the  various  tests  for  dextrose,  as  well  as  other 
which  have  the  same  property,  are  based. 

Fehling's  Test. — The  solution  usually  employed  for  both  qualitative  and 
quantitative  purposes  is  a  solution  of  cupric  hydroxid  made  alkaline  by  an 
excess  of  sodium  or  potassium  hydroxid,  with  the  addition  of  sodium  and 
potassium  tartrate.  This  solution,  originally  suggested  by  Fehling,  bears 
his  name.  It  is  made  by  dissolving  cupric  sulphate  34.  64  grams,  potassium 
hydroxid  125  grams,  sodium  and  potassium  tartrate  173  grams  in  1  liter  of 
distilled  water. 

The  reaction  is  expressed  by  the  following  equation: 

CuS04  +  2KOH  =  Cu(OH)2+ K2S04. 

The  object  of  the  sodium  and  potassium  tartrate  is  to  hold  the  Cu(OH)2 
in  solution.  If  a  few  cubic  centimeters  of  this  deep  blue  solution  be  boiled 
and  dextrose  then  added  and  the  solution  again  heated  to  the  boiling-point, 
the  cupric  hydroxid  is  reduced  to  the  condition  of  a  cuprous  oxid,  which 
shows  itself  as  a  red  or  orange-yellow  precipitate.  The  color  of  the^  pre- 
cipitate depends  on  the  relative  excess  of  either  copper  or  sugar,  being  red 
with  the  former,  orange  or  yellow  with  the  latter.  The  delicacy  of  this  test 
is  shown  by  the  fact  that  a  few  minims  of  this  solution  will  detect  in  one  c.c. 
of  water  the  1/ 15  of  a  milligram  of  sugar. 

For  quantitative  analysis,  ten  c.c.  of  Fehling's  solution,  diluted  with 
forty  c.c.  of  water,  are  heated  in  a  porcelain  capsule,  to  which  the  dextrose 
solution  is  cautiously  added  from  a  buret  until  the  blue  color  entirely  dis- 
appears. The  strength  of  this  solution  is  such  that  10  c.c.  is  decolorized 
by  50  milligrams  of  sugar,  from  which  the  percentage  of  sugar  in  the  urine 
can  be  determined.  Thus  if  0.8  c.c.  of  urine  decolorizes  10  c.c.  of  Fehling's 
solution  then  it  contains  50  milligrams  of  sugar. 

Fermentation  Test. — If  to  a  solution  of  dextrose  a  small  quantity  of  the 
yeast  plant  be  added,  and  the  solution  kept  at  a  temperature  of  250  C,  it 
will  gradually  undergo  fermentation;  that  is,  will  be  reduced  to  simpler 


CHEMIC    COMPOSITION    OF    THE   HUMAN    BODY.  II 

compounds  and  especially  to  alcohol  and  carbon  dioxid.     The  change  is 
expressed  in  the  following  equation: 

C6H1206  =  2C2H80  +  2C02. 
Dextrose.     Alcohol.      Carbon 

Dioxid.  • 

About  ninety-five  per  cent,  of  the  dextrose  is  so  changed,  the  remaining 
five  per  cent,  yielding  secondary  products  — succinic  acid,  glycerin,  etc. 

Levulose,  or  fruit-sugar,  is  found  in  association  with  dextrose  as  a 
constituent  of  many  fruits.  It  is  sweeter  than  dextrose  and  more  soluble 
in  both  water  and  dilute  alcohol.  From  alcoholic  solutions  it  crystallizes 
in  fine,  silky  needles,  though  it  usually  occurs  in  the  form  of  a  syrup. 

Levulose  is  distinguished  from  dextrose  by  its  property  of  turning  the 
plane  of  polarized  light  to  the  left;  the  extent  to  which  it  does  so,  however, 
varies  with  the  temperature  and  concentration  of  the  solution. 

Under  the  influence  of  the  yeast  plant  it  slowly  undergoes  fermentation, 
yielding  the  same  products  as  dextrose.  It  also  has  a  reducing  action  on 
cupric  oxid. 

Galactose  is  obtained  by  boiling  milk-sugar  (lactose)  with  dilute  sulphuric 
acid.  In  many  chemic  relations  it  resembles  dextrose.  It  is  less  soluble  in 
water,  however,  crystallizes  more  easily,  and  has  a  greater  dextro-rotary 
power.     It  also  undergoes  fermentation  with  the  yeast  plant. 

3.  SACCHAROSES,  C12H22On. 

Saccharose,  or  cane-sugar,  is  widely  distributed  throughout  the  vege- 
table world,  but  is  especially  abundant  in  sugar-cane,  sorghum  cane,  sugar- 
beet,  Indian  corn,  etc.  It  crystallizes  in  large  monoclinic  prisms.  It  is 
soluble  in  water  and  in  dilute  alcohol.  Saccharos  has  no  reducing  power 
on  cupric  oxid,  and  hence  its  presence  cannot  be  detected  by  Fehling's 
solution.  It  is  dextro-rotary.  Boiled  with  dilute  mineral,  as  well  as  organic 
acids,  saccharose  combines  with  water,  and  undergoes  some  change  in 
virtue  of  which  it  rotates  the  plane  of  polarized  light  to  the  left,  and  hence 
the  product  is  termed  invert  sugar.  This  latter  has  been  shown  to  be  a 
mixture  of  equal  quantities  of  levulose  and  dextrose.  This  inversion  of 
saccharose  through  hydration  and  decomposition  is  expressed  by  the  follow- 
ing equation: 

Ci2H22On  +  H20  =  C6H1206  +  C6H1206. 
Saccharose.    Water.    Levulose.      Dextrose. 


Invert  Sugar. 
Saccharose  is  not  directly  fermentable  by  yeast,  but  through  the  specific 
action  of  a  ferment,  invertin  or  invertase,  secreted  by  the  yeast  plant,  or  the 


12  HUMAN  PHYSIOLOGY. 

inverting  ferment  of  the  small  intestine,  it  undergoes  inversion,  as  previously 
stated,  after  which  it  is  readily  fermented,  yielding  alcohol  and  carbon  dioxid. 

Lactose  is  the  form  of  sugar  found  exclusively  in  the  milk  of  the  mammalia, 
from  which.it  can  be  obtained  in  the  form  of  hard,  white,  rhombic  prisms 
united  with  one  molecule  of  water.  It  is  souble  in  water,  insoluble  in  alcohol 
and  ether.  It  is  dextro-rotary.  It  requires  cupric  oxid,  but  to  a  less  extent 
than  dextrose.  Dilute  acids  decompose  it  into  equal  quantities  of  dextrose 
and  galactose.  Lactose  is  not  fermentable  with  yeast,  but  in  the  presence 
of  the  lactic  acid  bacillus  it  is  decomposed  into  lactic  acid,  and  finally  into 
butyric  acid,  as  follows: 

Ci2H22Ol1+H20=4C3H603 

Lactose.         Water.      Lactic  Acid. 


2C3H603    -    C4H802     + 

2C02 

+        2H2 

Lactic  Acid.  Butyric  Acid. 

Carbon 

Free 

Dioxid. 

Hydrogen. 

Maltose  is  a  transformation  product  of  starch,  and  arises  whenever 
the  latter  is  acted  on  by  malt  extract  or  the  diastatic  ferments  in  saliva  and 
pancreatic  juice.  It  can  also  be  produced  by  the  action  of  dilute  sulphuric 
acid  on  starch.     The  change  is  expressed  by  the  following  equation: 

2CflH10O6  +  H2O  =  C12H22Ou. 

Starch.        Water.      Maltose. 

Maltose  crystallizes  in  the  form  of  white  needles,  which  are  soluble  in  water 
and  in  dilute  alcohol.  It  is  dextro-rotary.  In  the  presence  of  ferments 
and  dilute  acids  maltose  undergoes  hydration  and  decomposition,  giving 
rise  to  two  molecules  of  dextrose.  It  has  a  reducing  action  on  cupric  oxid. 
Fermentation  is  readily  caused  by  yeast,  but  whether  directly  or  indirectly 
by  inversion  is  somewhat  uncertain. 

Osazones. — All  the  sugars  which  possess  the  power  of  reducing  cupric  oxid 
are  capable  of  combining  with  phenyl-hydrazin,  with  the  formation  of  com- 
pounds termed  osazones.  The  osazones  so  formed  are  crystalline  in  struc- 
ture, but  have  different  melting  points,  varying  degrees  of  solubility  and 
optic  properties,  all  of  which  serve  to  detect  the  various  sugars  and  to  dis- 
tinguish one  from  the  other.  Of  the  different  osazones,  phenyl-glucosazone 
is  the  most  characteristic,  and  occurs  in  the  form  of  long,  yellow  needles.  It 
may  be  obtained  from  dextrose  by  the  following  method:  To  fifty  c.c.  of  a 
dextrose  solution  add  2  gm.  of  phenyl-hydrazin  and  two  gm.  of  sodium 
acetate,  and  boil  for  an  hour.  On  cooling,  the  osazone  crystallizes  in  the 
form  of  long,  yellow  needles. 


CHEMIC   COMPOSITION    OF   THE   HUMAN   BODY.  13 

THE  FATS. 

The  fats  constitute  a  group  of  organic  bodies  found  in  the  tissues  of  both 
vegetables  and  animals.  In  the  vegetable  world  they  are  largely  found  in 
fruits,  seeds,  and  nuts,  where  they  probably  originate  from  a  transformation 
of  the  carbohydrates.  In  the  animal  body  the  fats  are  found  largely  in  the 
subcutaneous  tissue,  in  the  marrow  of  bones,  in  and  around  various  internal 
organs  and  in  milk.  In  these  situations  fat  is  contained  in  small,  round  or 
polygon-shaped  vesicles,  which  are  united  by  areolar  tissue  and  surrounded 
by  blood-vessels.  At  the  temperature  of  the  body  the  fat  is  liquid,  but  after 
death  it  soon  solidifies  from  the  loss  of  heat. 

The  fats  are  compounds  consisting  of  carbon,  hydrogen,  and  oxygen. 
The  percentage  composition  of  fat  (stearin)  is  as  follows:  carbon  76.86,  hy- 
drogen 12.36  oxygen  10.78.  The  fat,  as  found  in  animals,  is  a  mixture,  in 
varying  proportions  in  different  animals,  of  three  neutral  fats — stearin,  pal- 
mitin,  and  olein.  Each  fat  is  a  derivative  of  glycerin  and  the  particular  acid 
indicated  by  its  name — e.  g.,  stearic  acid,  in  the  case  of  stearin,  etc.  The  re- 
action which  takes  place  in  the  combination  of  glycerin  and  the  acid  is  ex- 
pressed in  the  following  equation: 

C3H5(HO)3  +  (HC18H3502)3  =  C3H5(C18H3502)3  +  3H20. 
Glycerin.  Stearic  Acid.  Stearin.  Water. 

Hence,  strictly  speaking,  the  fats  are  compound  ethers,  in  which  the 
hydrogen  of  the  organic  acid  is  replaced  by  the  trivalent  radicle,  tritenyl, 

C3H6- 

Stearin,  C3H5(C18H3502)3,  is  the  chief  constituent  of  the  more  solid  fats. 
It  is  solid  at  ordinary  temperatures,  melting  at  550  C,  then  solidifying  again 
as  the  temperature  rises,  until  at  710  C.  it  melts  permanently.  It  crystal- 
lizes in  square  tables. 

Palmitin,  C3H6(C16H3102)3,  is  a  semifluid  fat,  solid  at  450  C.  and  melting 
at  620  C.     It  crystallizes  in  fine  needles,  and  is  soluble  in  ether. 

Olein,  C3Hs(C18H3302)3,  is  a  colorless,  transparent  fluid,  liquid  at  ordi- 
nary temperatures,  only  solidifying  at  o°  C.  It  possesses  marked  solvent 
powers,  and  holds  stearin  and  palmitin  in  solution  at  the  temperature  of 
the  body. 

Saponification. — When  subjected  to  the  acton  of  superheated  steam, 
a  neutral  fat  is  saponified — i.  e.,  decomposed  into  glycerin  and  the  particular 
acid  indicated  by  the  name  of  the  fat  used:  e.g.,  stearic,  palmitic,  or  oleic. 
The  reaction  is  expressed  as  follows: 

C3H6(C18H3302)3  +  3H20=C3H6(HO)3  +  3(C18H3402). 
Olein.  Water.         Glycerin.        Oleic  Acid. 


14  HUMAN  PHYSIOLOGY. 

The  fatty  acids  thus  obtained  are  characterized  by  certain  chemic  features? 
as  follows: 

Stearic  acid  is  a  firm,  white  solid,  fusible  at  6o°  C.  It  is  soluble  in  ether 
and  alcohol,  but  not  in  water. 

Palmitic  acid  occurs  in  the  form  of  white,  glistening  scales  or  needles, 
melting  at  620  C. 

Oleic  acid  is  a  clear,  colorless  liquid,  tasteless  and  odorless  when  pure. 
It  crystallizes  in  white  needles  at  o°  C. 

If  this  saponification  take  place  in  the  presence  of  an  alkali — e.  g.,  potas- 
sium hydroxid,  sodium  hydroxid — the  acid  produced  combines  at  once  with 
the  alkali  to  form  a  salt  known  as  a  soap,  while  the  glycerin  remains  in  solu- 
tion.    The  reaction  is  as  follows: 

3KHO+(C18H3402)3  =  3(KC18H3302)+3H20. 
Potassium.    Oleic  Acid.     Potassium  Oleate.    Water. 

All  soaps  are,  therefore,  salts  formed  by  the  union  of  alkalies  and  fat 
acids.  The  sodium  soaps  are  generally  hard,  while  the  potassium  soaps  are 
soft.  Those  made  with  stearin  and  palmitm  are  harder  than  those  made 
with  olein.  If  the  soap  is  composed  of  lead,  zinc,  copper,  etc.,  it  is  insoluble 
jn  water. 

Emulsification. — When  a  neutral  oil  is  vigorously  shaken  with  water  or 
other  fluid,  it  is  broken  up  into  minute  globules  that  are  more  or  less  per- 
manently suspended;  the  permanency  depending  on  the  nature  of  the  liquid. 
The  most  permanent  emulsions  are  those  made  with  soap  solutions.  The 
process  of  emulsification  and  the  part  played  by  soap,  can  be  readily  observed 
by  placing  on  a  few  cubic  centimeters  of  a  solution  of  sodium  carbonate 
0.25  per  cent,  of  a  small  quantity  of  a  perfectly  neutral  oil  to  which  has  been 
added  2  or  3  per  cent,  of  a  fat  acid.  The  combination  of  the  acid  and  the 
alkali  at  once  forms  a  soap.  The  energy  set  free  by  this  combination  rapidly 
divides  up  the  oil  into  extremely  minute  globules.  A  spontaneous  emulsion 
is  thus  formed. 

THE  PROTEINS. 

The  proteins  constitute  a  group  of  organic  bodies  which  are  found  in  both 
vegetable  and  animal  tissues.  Though  present  in  all  animal  tissues,  they  are 
especially  abundant  in  muscles  and  bones,  where  they  constitute  twenty 
per  cent,  and  thirty  per  cent,  respectively.  Though  genetically  related,  and 
possessing  many  features  in  common,  the  different  members  of  the  protein 
group  are  distinguished  by  characteristic  physical  and  chemic  properties. 


CHEMIC    COMPOSITION    OF    THE   HUMAN    BODY. 


15 


The  average  percentage  composition  of  several  proteins  is  shown  in  the 


following  analyses: 

C. 

Egg-albumin. .  .  52.9 
Serum-albumin  53.0 
Casein 53.3 


H. 

7.2 
6.8 
7.07 


Myosin 52.82  7. 11 


N.         O.        S. 

15.6  23.9  0.4      (Wurtz). 

16.0  22.29  1.77  (Hammersten). 

15.91  22.03  0.82   (Chittenden  and  Painter). 

16.77  21.90  1.27   (Chittenden  and  Cummins). 


The  molecular  composition  of  the  proteins  is  not  definitely  known,  and 
the  formulae  which  have  been  suggested  are  therefore  only  approximative. 
Leow  assigns  to  albumin  the  formula  C72H112N18022S,  while  Schutzen- 
berger  raises  the  numbers  to  C240H3g2Na5O75S3,  either  of  which  shows  that 
the  protein  molecule  is  extremely  complex. 

Structure  of  the  Protein  Molecule. — From  the  large  size  of  the  protein 
molecule  as  indicated  by  its  chemic  composition  it  might  be  inferred  that  its 
structure  was  equally  complex.  This  modern  investigation  has  shown  to 
be  the  case. 

When  any  one  of  the  typical  proteins,  found  in  animal  or  vegetable  tissues, 
is  hydrolyzed  by  acids,  alkalies  and  animal  ferments  under  appropriate  con- 
ditions, it  can  be  resolved  through  a  series  of  descending  stages  into  relatively 
simple  nitrogen-holding  bodies  termed  amino-acids  and  diamino-acids,  of 
which  somewhat  more  than  twenty  have  been  isolated  and  their  properties 
determined.  The  principal  amino-acids  are  as  follows:  Glycocoll,  alanin, 
leucin,  isoleucin,  amino-isovalerianic  acid,  serin,  aspartic  acid,  glutamic  acid, 
phenylalanin,  tyrosin,  prolin,  tryptophan.  The  principal  diamino-acids 
are  as  follows:  Ornithin,  lysin,  histidin,  arginin,  cystin. 

The  protein  molecule  is  therefore  structurally  complex.  The  manner  in 
which  these  elementary  compounds  are  arranged,  united  or  grouped  in  any 
given  protein,  is  practically  unknown.  More  or  less  successful  attempts 
have  been  made  at  the  reconstruction  of  the  protein  molecule  by  synthetic 
methods,  by  the  union  of  two  or  more  of  the  amino-acids.  A  number  of 
such  compounds  have  been  formed  by  the  union  of  from  two  to  ten  or  more 
amino-acids,  all  of  them  exhibiting  many  of  the  protein  reactions.  Such 
bodies  are  termed  polypeptids. 

Physical  Properties. — As  a  class  proteins  are  characterized  by  the  fol- 
lowing properties: 

1.  Indiffusibility. — None  of  the  proteins  normally  assumes  the  crystalline 
form,  and  hence  they  are  not  capable  of  diffusing  through  parchment  or 
an  animal  membrane.  Peptone,  a  product  of  the  digestion  of  proteins,  is 
an  exception  as  regards  its  diffusibility.     As  met  with  in  the  body,  all 


1 6  HUMAN  PHYSIOLOGY. 

proteins  are  amorphous,  but  vary  in  consistence  from  the  liquid  to  the 
solid  state.  The  colloid  character  of  the  proteins  permits  of  their  sepa- 
ration and  purification  from  crystalloid  diffusible  compounds  by  the  process 
of  dialysis. 

2.  Solubility. — Some  of  the  proteins  are  soluble  in  water,  others  in  solutions 
of  the  neutral  salts  of  varying  degrees  of  concentration,  in  strong  acids  and 
alkalies.     All  are  insoluble  in  alcohol  and  ether. 

3.  Coagulability. — Under  the  influence  of  heat  and  various  acids  and 
animal  ferments,  the  proteins  readily  pass  from  the  soluble  liquid  state  to 
the  insoluble  solid  state,  attended  by  a  permanent  alteration  in  their 
chemic  composition.  To  this  change  the  term  coagulation  has  been  given. 
The  various  proteins  not  only  coagulate  at  different  temperatures,  but 
with  different  chemic  reagents — distinctive  features  which  permit  not  only 
of  their  detection,  but  separation.  Proteins  are  capable  of  precipitation 
without  losing  their  solubility  by  ammonium  sulphate,  sodium  chlorid 
and  magnesium  sulphate. 

4.  Fermentability. — In  the  presence  of  specific  microorganisms — bacteria 
— the  proteins,  owing  to  their  complexity  and  instability,  are  prone  to 
undergo  disintegration  and  reduction  to  simpler  compounds.  This 
decomposition  or  putrefaction  occurs  most  readily  when  the  conditions 
most  favorable  to  the  growth  of  bacteria  are  present — viz.,  a  temperature 
varying  from  250  C.  to  400  C,  moisture  and  oxygen.  The  intermediate 
as  well  as  the  terminal  products  of  the  decomposition  of  the  proteins  are 
numerous,  and  vary  with  the  composition  of  the  protein  and  the  specific 
physiologic  action  of  the  bacteria.  Among  the  intermediate  products 
is  a  series  of  alkaloid  bodies,  some  of  which  possess  marked  toxic  prop- 
erties, known  as  ptomains.  The  toxic  symptoms  which  frequently  follow 
the  ingestion  of  foods  in  various  stages  of  putrefaction  are  to  be  attributed 
to  these  compounds.  The  terminal  products  are  represented  by  hydrogen 
sulphid,  ammonia,  carbon  dioxid,  fats,  phosphates,  nitrates,  etc. 

Color  Tests  for  Proteins. — When  proteins  are  present  in  solution,  they 
may  be  detected  by  the  following  color  reactions — viz. : 

1.  Xanthoproteic.  The  solution  is  boiled  with  nitric  acid  for  several  min 
utes,  when  the  protein  assumes  a  light  yellow  color.  After  the  solution 
has  cooled,  the  addition  of  ammonia  changes  the  color  to  an  orange  or  am- 
berred. 

2.  The  rose-red  reaction.  The  solution  is  boiled  with  acid  nitrate  of  mer- 
cury (Millon's  reagent)  for  a  few  minutes,  when  the  coagulated  protein 
turns  a  purple-red  color. 


CHEMIC   COMPOSITION   OF   THE  HUMAN   BODY.  1 7 

3.  The  blue- violet  reaction.  A  few  drops  of  copper  sulphate  solution  are 
first  added  to  the  protein  solution,  and  then  an  excess  of  sodium  hydroxid. 
A  blue-violet  color  is  produced,  which  deepens  somewhat  on  heating,  but 
no  further  change  ensues. 

The  proteins  found  in  the  animal  body,  though  possessing  many  features 
in  common,  are  nevertheless  characterized  by  certain  special  features  which 
not  only  serve  for  their  identification,  but  for  their  classification  into  well- 
defined  groups,  as  follows: 

SIMPLE  PROTEINS. 
PROTAMINS. 

These  proteins  are  derived  for  the  most  part  from  the  heads  of  sperma- 
tozooids  of  fish.  When  subjected  to  hydrolysis  they  can  be  resolved 
into  the  diamino  bodies,  lysin,  arginin  and  histidin  of  which  they  constitute 
about  90  per  cent,  and  a  small  number  of  amino-acids.  For  the  reason  that 
the  protamins  contain  practically  but  these  three  amino-bodies  they  are  re- 
garded as  the  simplest  of  all  proteins. 

HISTONS. — These  proteins  are  a  little  more  complex  than  the  protamins, 
and  less  complex  than  the  typical  proteins.  They  are  formed  in  combination 
with  nucleic  acid  in  spermatozoids,  in  red  corpuscles  and  various  tissues. 

ALBUMINS. 

The  members  of  this  group  are  soluble  in  water,  in  dilute  saline  solutions, 
and  in  saturated  solutions  of  sodium  chlorid  and  magnesium  sulphate.  They 
are  coagulated  by  heat,  and  when  dried  form  an  amber-colored  mass. 

(a)  Serum-albumin  is  found  in  blood,  lymph,  chyle,  tissue  fluids,  and 
milk.  It  is  obtained  readily  by  precipitation  from  blood-serum,  after 
the  other  proteins  have  been  removed,  on  the  addition  of  ammonium 
sulphate.  When  freed  from  saline  constituents,  it  presents  itself  as  a 
pale,  amorphous  substance,  soluble  in  water  and  in  strong  nitric  acid. 
It  is  coagulated  at  a  temperature  at  730  C,  as  well  as  by  varions  acids 
— e.  g.}  citric,  picric,  nitric,  etc.     It  has  a  rotary  power  of  —  62. 6°. 

(6)  Egg-albumin. — Though  not  a  constituent  of  the  human  body, 
egg-albumin  resembles  the  foregoing  in  many  respects.  When 
obtained  in  the  solid  form  from  the  white  of  the  egg,  it  is  a  yellow 
mass  without  taste  or  odor.  Though  similar  to  serum-albumin,  it 
differs  from  it  in  being  precipitated  by  ether,  in  coagulating  at  54°  C, 
and  in  having  a  lower  rotary  power,  —  35. 50. 
2 


1 8  HUMAN  PHYSIOLOGY. 

GLOBULINS. 

The  members  of  this  group  are  insoluble  in  water  and  in  saturated  solu- 
tions of  sodium  chlorid  and  magnesium  sulphate  and  ammonium  sulphate. 
They  are  soluble,  however,  in  dilute  saline  solutions — e.  g.,  sodium  chlorid 
(one  per  cent.),  potassium  chlorid,  ammonium  chlorid,  etc.  They  are  coag- 
ulated by  heat. 

(a)  Serum-globulin  or  Paraglobulin. — This  protein,  as  its  name 
implies,  is  found  in  blood-serum,  though  it  is  present  in  other  animal 
fluids.  When  precipitated  by  magnesium  sulphate  or  carbon  dioxid, 
it  presents  itself  as  a  flocculent  substance,  insoluble  in  water,  soluble 
in  dilute  acids  and  alkalies,  and  coagulating  at  750  C. 

(b)  Fibrinogen. — This  protein  is  found  in  blood  plasma  in  association 
with  serum-glob uHn  and  serum-albumin.  It  is  also  present  in  lymph- 
tissue  fluids  and  in  pathologic  transudates.  It  can  be  obtained  from 
blood-plasma  which  has  been  previously  treated  with  magnesium 
sulphate  on  the  addition  of  a  saturated  solution  of  sodium  chlorid. 
It  is  soluble  in  dilute  acids  and  alkalies,  and  coagulates  at  560  C. 

(c)  Myosinogen  or  Myogen. — This  protein  is  a  constituent  of  the 
protoplasm  of  the  muscle-fibers.  During  the  living  condition  it  is 
liquid,  but  after  death  it  readily  undergoes  decomposition  into  an 
insoluble  portion  known  as  myogen  fibrin.  It  is  soluble  in  dilute 
hydrochloric  acid  and  dilute  alkalies.     It  coagulates  at  560  C. 

(d)  Globin. — This  is  a  product  of  the  spontaneous  decomposition  of 
the  coloring  matter  of  the  blood — hemoglobin — and  arises  when  the 
latter  is  exposed  to  the  air. 

(e)  Crystallin  or  Globulin. — This  is  obtained  by  passing  a  stream  of 
CO 2  through  a  watery  extract  of  the  crystalline  lens. 

DERIVED  ALBUMINS  OR  ALBUMINATES. 

The  proteins  of  this  group  are  derived  from  both  native  albumins  and 
globulins  by  the  gradual  action  of  dilute  acids  and  alkalies,  and  may  be 
regarded  as  compounds  of  a  protein  with  an  acid  or  an  alkali. 

(a)  Acid-albumin. — This  is  formed  when  a  native  albumin  is  digested 
with  dilute  hydrochloric  acid  (0.2  per  cent.)  or  dilute  sulphuric  acid 
for  some  minutes.  It  is  precipitated  by  neutralization  with  sodium 
hydroxid  (0.1  per  cent,  solution).  After  the  precipitate  is  washed, 
it  is  found  to  be  insoluble  in  distilled  water  and  in  neutral  saline  solu- 
tions.    In  acid  solutions  it  is  not  coagulated  by  heat. 

(6)  Alkali-albumin. — This  is  formed  when  a  native  albumin  is  treated 
with  a  dilute  alkali — e.  g.,  0.1  per  cent,  of  sodium  hydroxid — for  five 


CHEMIC    COMPOSITION    OF    THE   HUMAN    BODY.  1 9 

or  ten  minutes.  On  careful  neutralization  with  dilute  hydrochloric 
acid,  it  is  precipitated.  It  is  also  insoluble  in  distilled  water  and  in 
alkaline  solutions;  it  is  not  coagulable  by  heat. 

COAGULATED  PROTEINS. 

Although  these  proteins  are  not  found  as  constituents  of  the  animal  organ- 
ism, they  possess  much  interest  on  account  of  their  relation  to  prepared  foods 
and  to  the  digestive  process.  They  are  produced  when  solutions  of  egg- 
albumin,  serum-albumin,  or  globulins  are  subjected  to  a  temperature  of 
ioo°  C.  or  to  the  prolonged  action  of  alcohol.  They  are  insoluble  in  water, 
in  dilute  acids,  and  in  neutral  saline  solutions.  In  this  same  group  may  be 
included  also  those  coagulated  proteins  which  are  produced  by  the  action  of 
animal  ferments  on  soluble  proteins — e.  g.,  fibrin,  myosin,  casein. 

(a)  Fibrin. — Fibrin  is  derived  from  one  of  the  blood  proteins,  fibrin- 
ogen. It  arises  from  the  combination  of  fibrinogen  and  thrombin. 
It  is  not  present  under  normal  circumstances  in  the  circulating  blood, 
but  makes  its  appearance  after  the  blood  is  withdrawn  from  the 
vessels  and  at  the  time  of  coagulation.  It  can  also  be  obtained  by 
whipping  the  blood  with  a  bundle  of  twigs,  on  which  it  accumulates. 
When  freed  from  blood  by  washing  under  water,  it  is  seen  to  consist 
of  bundles  of  white  elastic  fibers  or  threads.  It  is  insoluble  in  water, 
in  alcohol,  and  ether.  In  dilute  acids  it  swells,  becomes  transparent, 
and  finally  is  converted  into  acid-albumin.  In  dilute  alkalies  a 
similar  change  takes  place,  but  the  resulting  product  is  an  alkali- 
albumin.  Fibrin  possesses  the  property  of  decomposing  hydrogen 
dioxid,  H202 — i.  e.,  liberating  oxygen,  which  accumulates  in  the 
form  of  bubbles  on  the  fibrin.  On  incineration  fibrin  yields  an  ash 
which  contains  calcium  phosphate  and  magnesium  phosphate. 

(b)  Myosin. — Myosin  or  myogen  develops  in  muscles  after  death  and 
is  the  cause  of  the  stiffening  of  the  muscles.  It  has  been  regarded 
as  a  derivative  of  the  soluble  protein  myosinogen  alone,  but  there  is 
evidence  that  in  its  formation  both  paramyosinogen  and  myosinogen 
take  part.  It  is  not  definitely  known  whether  this  is  the  result  of  the 
action  of  a  special  ferment  or  not. 

(c)  Casein. — Casein  is  derived  from  the  chief  protein  of  milk — case- 
inogen — by  the  action  of  a  special  ferment  known  as  rennin  or  chy- 
mosin.     This  ferment  is  a  constituent  of  gastric  juice. 

PROTEOSES  AND  PEPTONES. 

During  the  progress  of  the  digestive  process,  as  it  takes  place  in  the  stomach 
and  intestines,  there  is  produced  by  the  action  of  the  gastric  and  pancreatic 


20  HUMAN  PHYSIOLOGY. 

juices,  out  of  the  proteins  of  the  food,  a  series  of  new  proteins,  knows  as 
proteoses  and  peptones.  The  chemic  properties  of  these  substances  will  be 
considered  in  connection  with  the  process  of  digestion. 

CONJUGATED  OR  COMBINED  PROTEINS. 

The  different  members  of  this  group  are  capable  of  being  decomposed 
by  chemic  methods  into  a  protein  and  a  non-protein  substance;  e:  g.,  a  color- 
ing matter,  a  carbohydrate,  or  a  nuclein.  The  chemic  character  of  the  non- 
protein substance  furnishes  the  basis  for  the  following  classification: 

CHROMO-PROTEINS. 

(a)  Hemoglobin. — Hemoglobin  is  the  coloring  matter  of  the  red 
corpuscles,  of  which  it  constitutes  about  30  per  cent,  of  the  total  weight. 
It  possesses  the  power  of  absorbing  oxygen  as  it  passes  through  the 
lung  capillaries  and  of  yielding  it  up  to  the  tissues  as  it  passes  through 
the  tissue  capillaries.  In  the  arterial  blood  it  is  known  as  oxyhemo- 
globin, and  in  the  venous  blood  as  dioxy-  or  reduced  hemoglobin. 
When  hydrolyzed  by  acids  or  alkalies,  hemoglobin  undergoes  a 
cleavage  into  a  protein,  globin,  and  a  pigment  hematin. 

(b)  Myohematin. — Myohematin  is  a  protein  supposed  to  be  present 
in  muscle.  It  has  never  been  isolated,  hence  its  chemic  features  are  un- 
known. Spectroscopic  examination  indicates  that  it  is  capable  of  ab- 
sorbing and  again  yielding  up  oxygen.  For  this  reason  it  is  believed 
to  be  a  derivative  of  hemoglobin. 

GLUCO-PROTEINS. 

(a)  Mucin. — Mucin  is  the  protein  which  gives  the  mucus  secreted  by 
the  epithelial  cells  of  the  mucous  membranes  and  related  glands  its 
viscid,  tenacious  character.  It  is  also  a  constituent  of  the  intercellular 
substances  of  the  connective  tissues.  It  is  readily  precipitated  by 
acetic  acid.  When  heated  with  dilute  acids,  mucin  undergoes  a 
cleavage  into  a  similar  proteid  and  a  carbohydrate  termed  mucose, 
which  is  capable  of  reducing  Fehling's  solution. 

(b)  Mucoids. — The  mucoids  resemble  the  mucins  though  differing 
from  them  in  solubility  and  in  not  being  precipitable  from  alkaline 
solutions  by  acetic  acid.  They  are  found  in  the  vitreous  humor, 
white  of  egg,  cartilage,  and  in  other  situations.  They  differ  slightly 
one  from  the  other  in  properties  and  chemic  composition.  They 
yield  on  decomposition  a  carbohydrate. 


CHEMIC    COMPOSITION    OF    THE   HUMAN    BODY.  21 

NUCLEO-PROTEINS. 

The  nucleo-proteins  are  obtained  from  the  nuclei  and  cell-substance 
of  tissue-cells.  Chemically  they  are  characterized  by  the  presence  of  phos- 
phorus in  relatively  large  amounts.  When  hydrolyzed,  they  separate  into  a 
protein  and  a  nuclein. 

The  nucleins  derived  from  cell  nuclei  can  be  still  further  separated  into 
a  simpler  protein  and  nucleic  acid,  which  latter  in  turn  yields  phosphoric 
acid  and  the  so-called  purin  bases,  xanthin,  hypoxanthin,  adenin,  and 
guanin.     All  nucleins  which  yield  the  purin  bases  are  termed  true  nucleins. 

PHOSPHO-PROTEINS. 

The  two  members  of  this  group  are  distinguished  by  yielding  on  decom- 
position a  protein  which  contains  phosphorus.  They  do  not  give  rise  to 
purin  bases  as  they  at  one  time  were  supposed  to  do. 

(a)  Caseinogen. — This  is  the  principal  protein  of  milk,  in  which  it 
exists  in  association  with  an  alkali,  and  hence  was  formerly  regarded 
as  an  alkali-albumin.  It  is  precipitated  by  acetic  acid  and  by  mag- 
nesium sulphate.  It  is  coagulated  by  rennet — that  is,  changed  into 
an  insoluble  protein,  casein  or  tyrein.  Calcium  phosphate  seems  to 
be  the  natural  alkali  necessary  to  this  process,  for  if  it  be  removed  by 
dialysis,  or  precipitated  by  the  addition  of  potassium  oxalate,  coagula- 
tion does  not  take  place. 

(b)  Vitellin. — Vitellin  is  a  constituent  of  the  vitellis  or  yolk  of  eggs. 
It  differs  from  other  proteids  in  the  fact  that  it  is  semicrystalline 
in  character.  Though  usually  regarded  as  a  nucleo-proteid  it  is 
not  definitely  known  kwhether  or  not  it  contains  phosphorus  in  its 
composition. 

SCLERO-PROTEINS  (ALBUMINOIDS). 

The  albuminoids  constitute  a  group  of  substances  similar  to  the  proteins 
in  many  respects,  though  differing  from  them  in  others.     When  obtained 
from  the  tissues,  in  which  they  form  the  organic  basis,  they  are  found  to  be 
amorphous,  colloid,  and  when  decomposed  yield  products  similar  to  those 
of  the  true  proteins.     The  principal  members  of  the  group  are  as  follows: 
(a)  Collagen,  Ossein. — These  are  two  closely  allied,  if  not  identical,  sub- 
stances, found  respectively  in  the  white  fibrous  connective  tissue  and 
in  bone.     When  the  tendons  of  muscles,  the  ligaments,  or  decalcified 
bone  are  boiled  for  several  hours,  the  collagen  and  ossein  are  converted 
into  soluble  gelatin,  which,  when  the  solution  cools,  becomes  solid. 
(6)  Chondrigen. — This  is  supposed  to  be  the  organic  basis  of  the 
more  permanent  cartilages.     When  they  are  boiled,   they  yield  a 


2  2  HUMAN  PHYSIOLOGY. 

substance   which  gelatinizes  on   cooling,   and   to  which   the  name 
chondrin  has  been  given. 

(c)  Elastin  is  the  name  given  to  the  substance  composing  the  fibers 
of  the  yellow,  elastic  connective  tissue. 

(d)  Keratin  is  the  substance  found  in  all  horny  and  epidermic  tissues, 
such  as  hairs,  nails,  scales,  etc.  It  differs  from  most  proteins  in 
containing  a  high  percentage  of  sulphur. 

INORGANIC  CONSTITUENTS. 

The  inorganic  compounds  and  mineral  constituents  obtained  from  the 
solids  and  fluids  of  the  body  are  very  numerous,  and,  in  some  instances, 
quite  abundant.  Though  many  of  the  compounds  thus  obtained  are 
undoubtedly  derivatives  of  the  tissues  and  necessary  to  their  physical  and 
physiologic  activity,  others,  in  all  probability,  are  decomposition  products, 
or  transitory  constituents  introduced  with  the  food.  Of  the  inorganic  com- 
pounds, the  following  are  the  most  important: 

WATER. 

Water  is  the  most  important  of  the  inorganic  constituents,  as  it  is  in- 
dispensable to  life.  It  is  present  in  all  the  tissues  and  fluids  without  excep- 
tion, varying  from  99  per  cent,  in  the  saliva  to  80  per  cent,  in  the  blood,  7  5 
per  cent,  in  the  muscles  to  2  per  cent,  in  the  enamel  of  the  teeth.  The 
total  quantity  contained  in  a  body  weighing  75  kilograms  (165  pounds)  is 
52.  5  kilograms  (115  pounds).  Much  of  the  water  exists  in  a  free  condition, 
and  forms  the  chief  part  of  the  fluids,  giving  to  them  their  characteristic 
degree  of  fluidity.  Possessing  the  capability  of  holding  in  solution  a  large 
number  of  inorganic  as  well  as  some  organic  compounds,  and  being  at  the 
same  time  diffusible,  it  renders  an  interchange  of  materials  between  all 
portions  of  the  body  possible.  It  aids  in  the  absorption  of  new  material  into 
the  blood  and  tissues,  and  at  the  same  time  it  transfers  waste  products 
from  the  tissues  to  the  blood,  from  which  they  are  finally  eliminated,  along 
with  the  water  in  which  they  are  dissolved.  A  portion  of  the  water  is  chem- 
ically combined  with  other  tissue  constituents,  and  gives  to  the  tissues 
their  characteristic  physical  properties.  The  consistency,  elasticity,  and 
pliability  are,  to  a  large  extent,  conditioned  by  the  amount  of  water  they 
contain.  The  total  quantity  of  water  eliminated  by  the  kidneys,  lungs, 
and  skin  amounts  to  about  three  kilograms  (6|  pounds). 

CALCIUM  COMPOUNDS. 

Calcium  phosphate,  Ca3(P04)2,  has  a  very  extensive  distribution 
throughout  the  body.     It  exists  largely  in  the  bones,  teeth,  and  to  a  slight 


CHEMIC   COMPOSITION    OF   THE  HUMAN   BODY.  23 

extent  in  cartilage,  blood,  and  other  tissues.  Milk  contains  0.27  per  cent. 
The  solidity  of  the  bones  and  teeth  is  almost  entirely  due  to  the  presence 
of  this  salt,  and  is,  therefore,  to  be  regarded  as  necessary  to  their  structure. 
It  enters  into  chemic  union  with  the  organic  matter,  as  shown  by  the  fact 
that  it  can  not  be  separated  from  it  except  by  chemic  means,  such  as  hydro- 
chloric acid.  Though  insoluble  in  water,  it  is  held  in  solution  in  the  blood 
and  milk  by  the  protein  constituents,  and  in  the  urine  by  the  acid  phosphate 
of  soda.  The  total  quantity  of  calcium  phosphate  which  enters  into  the  for- 
mation of  the  body  has  been  estimated  at  2.5  kilograms.  The  amount 
eliminated  daily  from  the  body  has  been  estimated  at  0.4  gm.,  a  fact  which 
indicates  that  nutritive  changes  do  not  take  place  with  much  rapidity  in 
those  tissues  in  which  it  is  contained. 

Calcium  carbonate,  CaC03,  is  present  in  practically  the  same  situations 
in  the  body  as  the  phosphate,  and  plays  essentially  the  same  role.  It  is, 
however,  found  in  the  crystalline  form,  aggregated  in  small  masses  in  the 
internal  ear,  forming  the  otoliths,  or  ear  stones.  Though  insoluble,  it  is 
held  in  solution  by  the  carbonic  acid  diffused  through  the  fluids. 

Calcium  fluorid,  CaF2,  is  found  in  bones  and  teeth. 

SODIUM  COMPOUNDS. 

Sodium  fluorid,  NaCl,  is  present  in  all  the  tissues  and  fluids  of  the  body, 
but  especially  in  the  blood,  0.6  per  cent.;  lymph,  0.5,  and  pancreatic  juice, 
0.25  per  cent.  The  entire  quantity  in  the  body  has  been  estimated  at  about 
200  gm.  Sodium  chlorid  is  of  much  importance  in  the  body,  as  it  determines 
and  regulates  to  a  large  extent  the  phenomena  of  diffusion  which  are  there 
constantly  taking  place.  This  is  illustrated  by  the  fact  that  a  solution  of 
albumin  placed  in  the  rectum  without  the  addition  of  this  salt  will  not  be 
absorbed.  When  the  salt  is  added,  absorption  takes  place.  The  ingested 
water  is  absorbed  into  the  blood  largely  in  consequence  of  the  percentage 
of  this  salt  which  it  contains.  The  normal  percentage  of  sodium  chlorid  in 
the  blood-plasma  assists  in  maintaining  the  shape  and  structure  of  the  red 
blood-corpuscles  by  determining  the  amount  of  water  entering  into  their 
composition.     The  same  is  true  of  other  tissue  elements. 

Sodium  chlorid  also  influences  the  general  nutritive  process,  increasing 
the  disintegration  of  the  proteids,  as  shown  by  the  increased  amount  of  urea 
excreted.  During  its  existence  in  the  body  it  undergoes  some  chemic 
transformations  or  decompositions,  yielding  its  chlorid  to  form  potassium 
chlorid  of  the  blood-corpuscles  and  muscles  and  to  form  the  hydrochloric 
acid  of  the  gastric  juice. 

Sodium  phosphate,  Na2HP04,  is  found  in  all  solids  and  fluids  of  the 
body,   to  which,   with  but    few  exceptions,  it  imparts  an  alkaline  reaction. 


24  HUMAN  PHYSIOLOGY. 

This  is  especially  true  of  blood,  lymph,  and  tissue  fluids  generally.  It  is 
essential  to  physiologic  action  that  all  tissue  elements  should  be  bathed  by 
an  alkaline  medium. 

Sodium  carbonate,  Na2C03,  is  generally  found  in  association  with  the 
preceding  salt.  As  it  is  also  an  alkaline  compound,  it  assists  in  giving  to  the 
blood  and  lymph  their  characteristic  alkalinity.  In  carnivorous  animals 
the  sodium  phosphate  is  the  more  abundant,  while  in  the  herbivorous  animals 
the  reverse  is  true. 

Sodium  sulphate,  Na2S04,  is  present  in  many  of  the  tissues  and  fluids, 
especially  the  urine.  Though  introduced  in  the  food,  it  is  also,  in  all  prob- 
ability, formed  in  the  body  from  the  decomposition  and  oxidation  of  the 
proteins. 

POTASSIUM  COMPOUNDS. 

Potassium  chlorid,  KC1,  is  met  with  in  association  with  sodium  chlorid 
in  almost  all  situations  in  the  body.  It  preponderates,  however,  in  the 
tissue  elements,  especially  in  the  muscle  tissue,  nerve  tissue,  and  red  corpuscles. 
The  plasma  with  which  these  structures  are  bathed  contains  but  a  very  small 
amount  of  this  salt,  but,  as  previously  stated,  a  relatively  large  quantity  of 
sodium  chlorid.  Though  introduced  to  some  extent  in  the  food,  it  is  very 
likely  that  it  is  also  formed  through  the  decomposition  of  the  sodium  chlorid. 

Potassium  phosphate,  K2HP04,  is  found  in  association  with  sodium 
phosphate  in  all  the  fluids  and  solids.  As  it  has  similar  chemic  properties, 
its  functions  are  practically  the  same. 

Potassium  carbonate,  K2C03,  is  generally  found  with  the  preceding  salt. 

MAGNESIUM  COMPOUNDS. 

Magnesium  phosphate,  Mg3(P04)2,  is  found  in  all  tissues,  in  association 
with  calcium  phosphate,  though  in  much  smaller  quantity. 

Magnesium  carbonate,  MgCOs,  occurs  only  in  traces  in  the  blood. 

Both  of  these  compounds  have  functions  similar  to  the  calcium  compounds, 

and  exist,  in  all  probability,  under  similar  conditions. 

♦ 

IRON  COMPOUNDS. 

Iron  is  a  constituent  of  the  coloring  matter  of  the  blood.  Traces,  however, 
are  also  found  in  lymph,  bile,  gastric  juice,  and  in  the  pigment  of  the  eyes, 
skin,  and  hair.  The  amount  of  iron  contained  in  a  body  weighing  seventy 
kilograms  is  about  2.2  gm.  It  exists  under  various  forms — e.  g.,  ferric  oxid, 
ferrous  oxid,  and  in  combination  with  organic  compounds. 

Chemic  analysis  thus  shows  that  the  chemic  elements  into  which  the 
compounds  may  be  resolved  by  an  ultimate  analysis  do  not  exist  in  the  body 


PHYSIOLOGY    OF    THE    CELL.  25 

in  a  free  state,  but  only  in  combination,  and  in  characteristic  proportions, 
to  form  compounds  whose  properties  are  the  resultant  of  those  of  the  elements. 
Of  the  four  principal  elements  which  make  up  ninety-seven  per  cent,  of  the 
body,  O,  H,  N  are  extremely  mobile,  elastic,  and  possessed  of  great  atomic 
heat.  C,  H,  N  are  distinguished  for  the  narrow  range  of  their  affinities, 
and  for  their  chemic  inertia.  C  possesses  the  great  atomic  cohesion.  O  is 
noted  for  the  number  and  intensity  of  its  combinations. 

As  the  properties  of  the  compounds  formed  by  the  union  of  elements  must 
be  the  resultants  of  the  properties  of  the  elements  themselves,  it  follows  that 
the  ternary  compounds,  starches,  sugars,  and  fats  must  possess  more  or  less 
inertia,  and  at  the  same  time  instability;  while  in  the  more  complex  proteins, 
in  which  sulphur  and  phosphorus  are  frequently  combined  with  the  four 
principal  elements,  molecular  instability  attains  its  maximum.  As  all  the 
foregoing  compounds  possess  in  varying  degrees  the  properties  of  inertia  and 
instability,  it  follows  that  living  matter  must  possess  corresponding  properties, 
and  the  capability  of  undergoing  unceasingly  a  series  of  chemic  changes, 
both  of  composition  and  decomposition,  in  response  to  the  chemic  and  physical 
influences  by  which  it  is  surrounded,  and  which  underlie  all  the  phenomena 
of  life. 

PRINCIPLES  OF  DISSIMILATION. 

In  addition  to  the  previously  mentioned  compounds — viz.,  carbohydrates, 
fats,  proteins,  and  inorganic  salts — there  is  obtained  by  chemic  analysis 
from  the  tissues  and  fluids  of  the  body: 

1.  A  number  of  organic  acids,  such  as  acetic,  lactic,  oxalic,  butyric,  propionic, 
etc.,  in  combination  with  alkaline  and  earthy  bases. 

2.  Organic  compounds,  such  as  alcohol,  glycerin,  cholesterin. 

3.  Pigments,  such  as  those  found  in  bile  and  urine. 

4.  Crystallizable   nitrogenized   bodies,    such   as   urea,    uric   acid,  xanthin, 
hippuric  acid,  creatin,  creatinin,  etc. 

While  some  few  of  these  compounds  may  possibly  be  regarded  as  neces- 
sary to  the  physiologic  integrity  of  the  tissues  and  fluids,  the  majority  of 
them  are  to  be  regarded  as  products  of  dissimilation  of  the  tissues  and 
foods  in  consequence  of  functional  activity,  and  represent  stages  in  their 
reduction  to  simpler  forms  previous  to  being  eliminated  from  the  body. 

PHYSIOLOGY  OF  THE  CELL. 

A  histologic  analysis  of  the  tissues  shows  that  they  can  be  resolved  into 
simpler  elements,  termed  cells,  which  may,  therefore,  be  regarded  as  the 
primary  units  of  structure.     Though  cells  vary  considerably  in  shape,  size, 


26  HUMAN  PHYSIOLOGY. 

and  chemic  composition  in  the  different  tissues  of  the  adult  body,  they  are 
nevertheless,  descendants  from  typical  cells,  known  as  embryonic  or  undiffer 
entiated  cells,  the  first  offspring  of  the  fertilized  ovum.  Ascending  the  line 
of  embryonic  development,  it  will  be  found  that  every  organized  body  origi- 
nates in  a  single  cell — the  ovum.  As  the  cell  is  the  elementary  unit  of  all 
tissues,  the  function  of  each  tissue  must  be  referred  to  the  function  of  the  cell. 
Hence  the  cell  may  be  defined  as  the  primary  anatomic  and  physiologic 
unit  of  the  organic  world,  to  which  every  exhibition  of  life,  whether  normal 
or  abnormal,  is  to  be  referred. 

Structure  of  Cells. — Though  cells  vary  in  shape  and  size  and  internal 
structure  in  different  portions  of  the  body,  a  typical  cell  may  be  said  to  consist 
mainly  of  a  gelatinous  substance  forming  the  body  of  the  cell,  termed  cyto- 
plasm or  bioplasm,  in  which  is  embedded  a  smaller  spheric  body,  the  nucleus. 
Within  the  nucleus  there  is  frequently  a  still  smaller  body  the  nucleolus. 
The  shape  of  the  adult  cell  varies  according  to  the  tissue  in  which  it  is  found; 
when  young  and  free  to  move  in  a  fluid  medium,  the  cell  assumes  a  spheric 
form,  but  when  subjected  to  pressure,  may  become  cylindric,  fusiform, 
polygonal,  or  stellate.  Cells  vary  in  size  within  wide  limits,  ranging  from 
¥2Xo  o  °f  an  inch,  the  diameter  of  a  red  blood-corpuscle,  to  ^-^  of  an  inch, 
the  diameter  of  the  large  cells  in  the  gray  matter  of  the  spinal  cord.  (See 
Fig.  2). 

The  cell  cytoplasm  consists  of  a  soft,  semifluid,  gelatinous  material,  vary- 
ing somewhat  in  appearance  in  different  tissues.  Though  frequently  homo- 
geneous, it  often  exhibits  a  finely  granular  appearance  under  medium  powers 
of  the  microscope.  Young  cells  consist  almost  entirely  of  clear  cytoplasm, 
mature  cells  contain,  according  to  the  tissue  in  which  they  are  found,  material 
of  an  entirely  different  character — e.  g.,  small  globules  of  fat,  granules  of 
glycogen,  mucigen,  pigments,  digestive  ferments,  etc.  Under  high  powers 
of  the  microscope  the  cytoplasm  is  found  to  be  pervaded  by  a  network  of 
fibers,  termed  spongioplasm,  in  the  meshes  of  which  is  contained  a  clearer 
and  more  fluent  substance,  the  hyaloplasm.  The  relative  amount  of  these 
two  constituents  varies  in  different  cells,  the  proportion  of  hyaloplasm  being 
usually  greater  in  young  cells.  The  arrangement  of  the  fibers  forming  the 
spongioplasm  also  varies,  the  fibers  having  sometimes  a  radial  direction,  in 
others  a  concentric  disposition,  but  most  frequently  being  distributed  evenly 
in  all  directions.  In  many  cells  the  outer  portion  of  the  cell  cytoplasm  under- 
goes chemic  changes  and  is  transformed  into  a  thin,  transparent,  homogenous 
membrane — the  cell  membrane — which  completely  incloses  the  cell  substance. 
The  cell  membrane  is  permeable  to  water  and  watery  solutions  of  various  inor- 
ganic and  organic  substances.    It  is,  however,  not  an  essential  part  of  the  cell. 


PHYSIOLOGY   OF   THE   CELL. 


27 


The  nucleus  is  a  small  vesicular  body  embedded  in  the  cytoplasm  near 
the  center  of  the  cell.  In  the  resting  condition  of  the  cell  it  consists  of  a 
distinct  membrane,  composed  of  amphipyrenin,  inclosing  the  nuclear  contents. 
The  latter  consists  of  a  homogenous  amorphous  substance — the  nuclear 
matrix — in  which  is  embedded  the  nuclear  network.  It  can  often  be  seen 
that  a  portion  of  one  side  of  the  nucleus,  called  the  pole,  is  free  from  this  net- 
work. The  main  cords  of  the  network  are  arranged  as  V-shaped  loops  about 
it.     These  main  cords  send  out  secondary  branches  or  twigs,  which,  uniting 


Nuclear  mem- 
brane. 


Linin.    - 


Nuclear  fluid 
(matrix). 


Nucleolus 


Chromatin  cords 
(nuclear  network). 


Nodal  enlargement  " 
of  the  chromatin. 


Cell  membrane. 

—  Exoplasm. 

*,  Microsomes. 

Centrosome. 

Spongioplasm. 
r/ Hyaloplasm. 


-^is*- -    Foreign  inclo- 
sures. 


Fig.  2. — Diagram  of  a  Cell. 
Microsomes  and  spongioplasm  are  only  partly  drawn. 


with  one  another,  complete  the  network.  The  nuclear  cords  are  composed 
of  granules  of  chromatin — so  called  because  of  its  affinity  for  certain  staining 
materials — held  together  by  an  achromatin  substance  known  as  linen.  Besides 
the  nuclear  network,  there  are  embedded  in  the  nuclear  matrix  one  or  more 
small  bodies  composed  oipyrenin  known  as  nucleoli.  At  the  pole  of  the  nu- 
cleus, either  within  or  just  without  in  the  cytoplasm,  is  a  small  body,  the  cen- 
trosome, or  pole  corpuscle. 

Chemic  Composition  of  the  Cell. — The  composition  of  living  pro- 
toplasm is  difficult  of  determination,  for  the  reason  that  all  chemic  and  phys- 
ical methods  employed  for  its  analysis  destroy  its  vitality,  and  the  products 
obtained  are  peculiar  to  dead  rather  than  to  living  matter.     Moreover, 


28  HUMAN  PHYSIOLOGY. 

as  protoplasm  is  the  seat  of  constructive  and  destructive  processes,  it  is  not 
easy  to  determine  whether  the  products  of  analysis  are  crude  food  constit- 
uents or  cleavage  or  disintegration  products.  Nevertheless,  chemic  investi- 
gations have  shown  that  even  in  the  living  condition  protoplasm  is  a  highly 
complex  compound — the  resultant  of  the  intimate  union  of  many  different 
substances.  About  seventy-five  per  cent,  of  protoplasm  consists  of  water 
and  twenty-five  per  cent,  of  solids,  of  which  the  more  important  compounds 
are  various  nucleo-proteins  (characterized  by  their  large  percentage  of 
phosphorus),  globulins,  traces  of  lecithin,  cholesterin,  and  frequently  fat 
and  carbohydrates.  Inorganic  salts,  especially  the  potassium,  sodium, 
and  calcium  chlorids  and  phosphates,  are  almost  invariable  and  essential 
constituents. 

MANIFESTATIONS  OF  CELL  LIFE. 

Growth,  the  Maintenance  of  Nutrition  and  Reproduction. — All  cells 
exhibit  the  three  fundamental  properties  of  life — viz.,  growth,  the  maintenance 
of  nutrition  and  reproduction.  All  cells  when  newly  reproduced  are  ex- 
tremely small,  but  by  the  absorption  of  nutritive  material  from  their  sur- 
rounding medium,  they  gradually  grow  until  they  attain  their  mature  size. 
This  is  accomplished  by  the  power  which  living  material  possesses  of  trans- 
forming, vitalizing,  and  organizing  crude  nutritive  material,  through  a  series 
of  upward  changes,  into  material  similar  to  itself.  To  all  these  changes  the 
term  assimilation,  or  anabolism,  has  been  given.  Some  of  the  absorbed 
material,  in  all  probability,  never  becomes  an  integral  part  of  the  living 
bioplasm,  but  undergoes  disruption  and  oxidation,  giving  rise  at  once  to  heat. 
Coincident  with  the  assimilative  processes,  a  series  of  disintegrative  processes 
is  constantly  taking  place,  whereby  the  living  material  is  reduced,  through 
a  series  of  downward  chemic  changes,  to  simpler  compounds,  such  as  water, 
carbon  dioxid,  urea,  etc.  To  all  these  downward  changes  the  term  dis- 
similation, or  katabolism,  has  been  given.  As  a  result,  also,  of  these  various 
changes,  the  protoplasm  gives  rise  to  the  production  of  material  of  an  entirely 
different  character,  such  as  globules  of  fat,  granules  of  glycogen,  mucigen, 
digestive  ferments,  etc.  The  sum  total  of  all  changes  which  go  on  in  the  cell, 
both  assimilative  and  dissimilative,  are  embraced  under  the  general  term 
nutrition,  or  metabolism.  Every  cell  presents  in  its  nutritive  activities  an 
epitome  of  the  nutritive  activities  of  the  body  as  a  whole. 

Physiologic  Properties  of  Protoplasm. — All  living  protoplasm  pos- 
sesses properties  which  serve  to  distinguish  and  characterize  it — viz.,  irrita- 
bility, conductivity,  and  motility. 

Irritability,  or  the  power  of  reacting  in  a  definite  manner  to  some  form  of 


PHYSIOLOGY   OF   THE   CELL.  29 

external  excitation,  whether  mechanical,  chemic,  or  electric,  is  a  fundamental 
property  of  all  living  protoplasm.  The  character  and  extent  of  the  reaction 
will  vary,  and  will  depend  both  on  the  nature  of  the  protoplasm  and  the 
character  and  strength  of  the  stimulus.  If  the  protoplasm  be  muscle,  the 
response  will  be  a  contraction;  if  it  be  gland,  the  response  will  be  secretion; 
if  it  be  nerve,  the  response  will  be  a  sensation  or  some  other  form  of  nerve 
activity. 

Conductivity,  or  the  power  of  transmitting  molecular  disturbances  arising 
at  one  point  to  all  portions  of  the  irritable  material,  is  also  a  characteristic 
feature  of  all  protoplasm.  This  power,  however,  is  best  developed  in  that 
form  of  protoplasm  found  in  nerves,  which  serves  to  transmit,  with  extreme 
rapidity,  molecular  disturbances  arising  at  the  periphery  to  the  brain,  as 
well  as  in  the  reverse  direction.  Muscle  protoplasm  also  possesses  the 
same  power  in  a  high  degree. 

Motility,  or  the  power  of  executing  apparently  spontaneous  movements, 
is  exhibited  by  many  forms  of  cell  protoplasm.  In  addition  to  the  molecular 
movements  which  take  place  in  certain  cells,  other  forms  of  movement  are 
exhibited,  more  or  less  constantly,  by  many  cells  in  the  animal  body — e.  g., 
the  waving  of  cilia,  the  ameboid  movements  and  migrations  of  white  blood 
corpuscles,  the  activities  of  spermatozooids,  the  projections  of  pseudopodia, 
etc.  These  movements,  arising  without  any  recognizable  cause,  are  fre- 
quently spoken  of  as  spontaneous.  Strictly  speaking,  however,  all  proto- 
plasmic movement  is  the  resultant  of  natural  causes,  the  true  nature  of  which 
is  beyond  the  reach  of  present  methods  of  investigation. 

Reproduction. — Cells  reproduce  themselves  in  the  higher  animals  in 

two  ways — by  direct  division  and  by  indirect  division,  or  karyokinesis. 

In  the  former  the  nucleus  becomes  constricted,  and  divides  without  any  special 

grouping  of  the  nuclear  elements.     It  is  probable  that  this  occurs  only  in 

disintegrating  cells,  and  never  in  a  physiologic  multiplication.     In  division 

by  karyokinesis   (Fig.  3)   there  is  a  progressive  rearranging  and  definite 

grouping  of  the  nucleus,     the  result  of  which  changes  is  the  division  of  the 

centrosome,  the  chromatin,  and  the  rest  of  the  nucleus  into  two  equal  portions, 

which  form  the  nuclei.     Following  the  division  of  the  nuclei,  the  protoplasm 

divides.     The  process  may  be  divided  into  three  phases: 

1.  Prophase. — The  centrosome,  at  first  small  and  lying  within  the  nucleus, 

increases  in  size  and  moves  into  the  protoplasm,  where  it  lies  near  the 

nucleus,  surrounded  by  a  clear  zone,  from  which  delicate  threads  radiate 

through  an  area  known  as  the  attraction  sphere.     The  nucleus  enlarges  and 

becomes  richer  in  chromatin.     The  lateral  twigs  of  the  chromatin  cords 

are  drawn  in,  while  the  main  cords  become  much  contorted.     These  cords 


3° 


HUMAN  PHYSIOLOGY. 


have  a  general  direction  transverse  to  the  long  axis  of  the  cell,  and  parallel 
to  the  plane  of  future  cleavage.  They  are  seen  as  V-shaped  segments  or 
loops,  chromosomes,  having  their  closed  ends  directed  toward  a  common 
center,  the  polar  field,  while  the  other  ends  interdigitate  on  the  opposite 
side  of  the  nucleus — the  anti-pole.  The  polar  field  corresponds  to  the 
area  occupied  by  the  centrosome.     This  arrangement  is  known  as  the 


Close  Skein 
(viewed  from 
the  side). 
Polar  field. 


Loose  Skein  (viewed 
from  above — i.  e.,  from 
the  pole). 


Mother  Stars  (viewed  from  the  side) . 


(Mm  M$k 


i  m%p\ 


wteii-w^sila 


...      '   / 


Mother  Star  (viewed   Daughter  Star.         Beginning.  Completed. 

from  above).  Division  of  the  Protoplasm.^ 

Fig.  3. — Karyokinetic  Figures  Observed  in  the  Epithelium  op  the  Oral 

Cavity  of  a  Salamander. 

The  picture  in  the  upper  right-hand  corner  is  from  a  section  through  a  dividing 
egg  of  Siredon  pisciformis.  Neither  the  centrosomes  nor  the  first  stages  of  the 
development  of  the  spindle  can  be  seen  by  this  magnification.     X  560. 


close  skein;  but  as  the  process  goes  on,  the  chromosomes  become  thicker, 
shorter  and  less  contorted,  producing  a  much  looser  arrangement,  known 
as  the  loose  skein.  During  the  formation  of  the  loose  skein,  the  centro- 
some divides  into  two  portions,  which  move  apart  to  positions  at  the_  oppo- 
site ends  of  the  long  axis  of  the  nucleus.  At  the  same  time  delicate  achro- 
matin  fibers  make  their  appearance,  arranged  in  the  form  of  a  double  cone, 
the 'apices  of  which  correspond  in  position  to  the  centrosome.  This  is 
known  as  the  nuclear  spindle.  During  the  prophase  the  nuclear  mem- 
brane and  the  nucleoli  disappear. 


HISTOLOGY   OF   EPITHELIAL  AND    CONNECTIVE   TISSUES.      3 1 

2.  The  Metaphase. — The  two  centrosomes  are  at  opposite  ends  of  the  long 
avis  of  the  nucleus,  each  surrounded  by  an  attraction  sphere,  now  called 
the  polar  radiation.  The  chromosomes  become  yet  shorter  and  thicker, 
and  move  toward  the  equator  of  the  nucleus,  where  they  lie  with  their 
closed  ends  toward  the  axis,  presenting  the  appearance,  when  seen  from 
the  poles,  of  a  star — the  so-called  mother  star,  or  monaster.  While 
moving  toward  the  equator  of  the  nucleus,  and  often  earlier,  each  chromo- 
some undergoes  longitudinal  cleavage,  the  sister  loops  remaining  together 
for  a  time.  Upon  the  completion  of  the  monaster,  one  loop  of  each  pair 
passes  to  each  pole  of  the  nucleus,  guided,  and  perhaps  drawn  by  the 
threads  of  the  nuclear  spindle.  The  separation  of  the  sister  segments 
begins  at  their  apices,  and  as  the  open  ends  are  drawn  apart  they  remain 
connected  by  delicate  achromatin  filaments  drawn  out  from  the  chromo- 
somes. This  separation  of  the  daughter  chromosomes,  and  their  move- 
ment toward  the  daughter  centrosomes,  is  called  metakinesis.  As  they 
approach  their  destination,  we  have  the  appearance  of  two  stars  in  the 
nucleus — the  daughter  stars,  or  diasters. 

3.  Anaphase. — The  daughter  stars  undergo,  in  reverse  order,  much  the 
same  changes  that  the  mother  star  passed  through.  The  chromosomes 
become  much  convoluted,  and  perhaps  united  to  one  another,  the  lateral 
twigs  appear,  and  the  chromatin  resumes  the  appearance  of  the  resting 
nucleus.  The  nuclear  spindle,  with  most  of  the  polar  radiation,  disappears, 
and  the  nucleoli  and  the  nuclear  membrane  reappear,  thus  forming  two 
complete  daughter  nuclei.  Meanwhile  the  protoplasm  becomes  con- 
stricted midway  between  the  young  nuclei.  This  constriction  gradually 
deepens  until  the  original  cell  is  divided,  with  the  formation  of  two  com- 
plete cells. 

HISTOLOGY  OF  THE  EPITHELIAL  AND  CONNECTIVE 

TISSUES. 

1.  EPITHELIAL  TISSUE. 

The  epithelial  tissue  consists  of  one  or  more  layers  of  cells  resting  on  a 
homogeneous  membrane,  the  other  side  of  which  is  abundantly  supplied  with 
blood-vessels  and  nerves.  The  form  of  the  epithelial  cell  varies  in  different 
situations,  and  may  be  flattened,  cuboid,  spheroid,  or  columnar.  The 
form  of  the  cell  in  all  instances  is  related  to  some  specific  function.  When 
arranged  in  layers  or  strata,  the  cells  are  cemented  together  by  an  inter- 
cellular substance — mucin. 

The  epithelial  tissue  forms  a  continuous  covering  for  the  surfaces  of  the 


32  HUMAN  PHYSIOLOGY. 

body.  The  external  investment  (the  skin)  and  the  internal  investment 
(the  mucous  membrane,  which  lines  the  entire  alimentary  canal  and  its 
associated  body  cavities)  are  both  formed,  in  all  situations,  by  the  homo- 
geneous basement  membrane,  covered  with  one  or  more  layers  of  cells.  All 
materials,  therefore,  whether  nutritive,  secretory,  or  excretory,  must  pass 
through  epithelial  cells  before  they  can  enter  into  the  formation  of  the  blood 
or  be  eliminated  from  it.  The  nutrition  of  the  epithelial  tissue  is  maintained 
by  the  nutritive  material  derived  from  the  blood  diffusing  itself  into  and 
through  the  basement  membrane.  Chemically,  the  epithelial  cells  of  the 
epidermis — hair,  nails,  etc. — are  composed  of  an  albuminoid  material 
(keratin),  a  small  quantity  of  water,  and  inorganic  salts.  In  other  situatious, 
especially  on  the  mucous  membranes,  the  cells  consist  largely  of  mucin,  in 
association  with  other  proteins.  The  consistency  of  epithelium  varies  in 
accordance  with  external  influences,  such  as  the  presence  or  absence  of  mois- 
ture, pressure,  friction,  etc.  This  is  well  seen  in  the  skin  of  the  palms  of  the 
hands  and  the  soles  of  the  feet — situations  where  it  acquires  its  greatest 
density.  In  the  alimentary  canal,  in  the  lungs,  and  in  other  cavities,  where 
the  reverse  conditions  prevail,  the  epithelium  is  extremely  soft.  Epithelial 
tissues  aso  possess  varying  degrees  of  cohesion  and  elasticity — physical 
properties  which  enable  them  to  resist  considerable  pressure  and  distension 
without  having  their  physiologic  integrity  destroyed.  Inasmuch  as  these 
tissues  are  poor  conductors  of  heat,  they  assist  in  preventing  too  rapid  radia- 
tion of  heat  from  the  body,  and  cooperate  with  other  mechanisms  in  main- 
taining the  normal  temperature.  The  physiologic  activity  of  all  epithelial 
tissue  depends  on  a  due  supply  of  nutritive  material  derived  from  the  blood, 
which  not  only  maintains  its  own  nutrition,  but  affords  those  materials  out 
of  which  are  formed  the  secretions  of  the  glands,  whether  of  the  skin  or 
mucous  membrane. 

Functions  of  Epithelial  Tissue. — In  succeeding  chapters  the  form, 
chemic  composition,  and  functions  of  epithelial  cells  will  be  considered  in 
connection  with  the  functions  of  the  organs  of  which  they  constitute  a  part. 
In  this  connection  it  may  be  stated  in  a  general  way  that  the  functions  of  the 
epithelial  tissues  are: 

i.  To  serve  on  the  surface  of  the  body  as  a  protective  covering  to  the  under- 
lying structures  which  collectively  form  the  true  skin,  thus  protecting  them 
from  the  injurious  influences  of  moisture,  air,  dust,  microorganisms,  etc., 
which  would  otherwise  impair  their  vitality.  Wherever  continuous 
pressure  is  applied  to  the  skin,  as  on  the  palms  of  the  hands  and  soles  of 
the  feet,  the  epithelium  increases  in  thickness  and  density,  and  thus  pre- 
vents undue  pressure  on  the  nerves  of  the  true  skin.     The  density  of  the 


HISTOLOGY    OF   EPITHELIAL  AND    CONNECTIVE   TISSUES.      33 

epidermis  enables  it  to  resist,  within  limits,  the  injurious  influences  of 
acids,  alkalies,  and  poisons. 

To  promote  absorption.  Inasmuch  as  the  skin  and  mucous  membranes 
cover  the  surfaces  of  the  body,  it  is  obvious  that  all  nutritive  material 
entering  the  body  must  first  traverse  the  epithelial  tissue.  Owing  to 
their  density,  however,  the  epithelial  cells  covering  the  skin  play  but  a 
feeble  role  as  absorbing  agents  in  man  and  the  higher  animals.  The 
epithelium  of  the  mucous  membrane  of  the  alimentary  canal,  particularly 
that  of  the  small  intestine,  is  especially  adapted,  from  its  situation,  con- 
sistency, and  properties,  to  play  the  chief  role  in  the  absorption  of  new 
materials  into  the  blood.  The  epithelium  lining  the  air-vesicles  of  the 
lungs  is  engaged  in  promoting  the  absorption  of  oxygen  and  the  exhalation 
of  carbon  dioxid. 

To  form  secretions  and  excretions.  Each  secretory  gland  connected  with 
the  surfaces  of  the  body  is  lined  by  epithelial  cells,  which  are  actively  con- 
cerned in  the  formation  of  the  secretion  peculiar  to  the  gland.  Each 
excretory  organ  is  similarly  provided  with  epithelial  cells,  which  are  en- 
gaged either  in  the  production  of  the  constituents  of  the  excretion  or  in 
their  removal  from  the  blood. 


2.  THE  CONNECTIVE  TISSUES. 

The  connective  tissues,  in  their  collective  capacity,  constitute  a  frame- 
work which  pervades  the  body  in  all  directions,  and,  as  the  name  implies, 
serve  as  a  bond  of  connection  between  the  individual  parts,  at  the  same  time 
affording  a  basis  of  support  for  the  muscle,  nerve,  and  gland  tissues.  The 
connective-tissue  group  includes  a  number  of  varieties,  among  which  may 
be  mentioned  the  areolar,  adipose,  retiform,  white  fibrous,  yellow  elastic, 
cartilaginous  and  osseous.  Notwithstanding  their  apparent  diversity,  they 
possess  many  points  of  similarity.  They  have  a  common  origin,  developing 
from  the  same  embryonic  material;  they  have  much  the  same  structure, 
passing  imperceptibly  into  one  another,  and  perform  practically  the  same 
functions. 

Areolar  Tissue. — This  variety  is  found  widely  distributed  throughout 
the  body.  It  serves  to  unite  the  skin  and  mucous  membrane  to  the  struc- 
tures on  which  they  rest;  to  form  sheaths  for  the  support  of  blood-vessels, 
nerves,  and  lymphatics;  to  unite  into  compact  masses  the  muscular  tissue  of 
the  body,  etc.  Examined  with  the  naked  eye,  it  presents  the  appearance 
of  being  composed  of  bundles  of  fine  fibers  interlacing  in  every  direction. 

3 


34  HUMAN  PHYSIOLOGY. 

In  the  embryonic  state  the  elements  of  this  form  of  connective  tissue  are 
united  by  a  ground  substance,  gelatinous  in  character.  In  the  adult  state 
this  substance  shrinks  and  largely  disappears,  leaving  intercommunicating 
spaces  of  varying  size  and  shape,  from  which  the  tissue  takes  its  name.  When 
subjected  to  the  action  of  various  reagents,  and  examined  microscopically, 
the  bundles  can  be  shown  to  consist  of  extremely  delicate,  colorless,  trans- 
parent, wavy  fibers,  which  are  cemented  together  by  a  ground  substance 
composed  largely  of  mucin.  Other  fibers  are  also  observed,  which  are  dis- 
tinguished by  a  straight  course,  a  sharp,  well-defined  outline,  a  tendency  to 
branch  and  unite  with  adjoining  fibers,  and  to  curl  up  at  their  extremities 
when  torn.  From  their  color  and  elasticity  they  are  known  as  yellow  elastic 
fibers.  Distributed  throughout  the  meshes  of  the  areolar  tissue  are  found 
flattened,  irregularly  branched,  or  stellate  corpuscles,  connective-tissue 
corpuscles,  plasma  cells,  and  granule  cells. 

Adipose  Tissue. — This  tissue,  which  exists  very  generally  throughout 
the  body,  though  found  most  abundantly  beneath  the  skin,  around  the 
kidneys,  and  in  the  bones,  is  practically  but  a  modification  of  areolar  tissue. 
In  these  situations  it  presents  itself  in  small  masses  or  lobules  of  varying 
size  and  shape,  surrounded  and  penetrated  by  the  fibers  of  connective  tissue. 
Microscopic  examination  shows  that  these  masses  consist  of  small  vesicles  o 
cells,  round,  oval,  or  polyhedral  in  shape,  depending  somewhat  on  pressure. 
Each  vesicle  consists  of  a  thin,  colorless,  protoplasmic  membrane,  thickened 
at  one  point,  in  which  a  nucleus  can  usually  be  detected.  This  membrane 
incloses  a  globule  of  fat,  which  during  life  is  in  the  liquid  state.  It  is  com- 
posed of  olein,  stearin,  and  palmitin.  The  origin  of  the  fat  is  to  be  referred 
to  a  retrograde  change  in  the  protoplasmic  material  of  the  connective-tissue 
cells.  When  this  protoplasm  becomes  rich  in  carbon  and  hydrogen,  it  is 
speedily  converted  into  fat,  which  makes  its  appearance  in  the  form  of  minute 
drops  in  different  portions  of  the  cell.  As  the  drops  accumulate,  at  the  ex- 
pense of  the  cell  protoplasm  they  gradually  coalesce,  until  there  remains 
but  a  thin  stratum  of  the  protoplasm,  which  forms  the  wall  of  the  vesicle. 
Adipose  tissue  may,  therefore,  be  regarded  as  areolar  tissue,  in  which  and  at 
the  expense  of  some  of  its  elements,  fat  is  stored  for  the  future  needs  of  the 
organism.  A  diminution  of  food,  especially  of  fat  and  carbohydrates,  is 
promptly  followed  by  an  absorption  of  fat  by  the  blood-vessels  and  by  its 
transference  to  the  tissues,  where  it  is  either  utilized  for  tissue  construction 
or  for  oxidation  purposes.  In  the  situations  in  which  adipose  tissue  is  found 
it  seems,  by  its  chemic  and  physical  properties,  to  assist  in  the  prevention  of  a 
too  rapid  radiation  of  heat  from  the  body,  to  give  form  and  roundness,  and 
to  diminish  angularities,  etc. 


HISTOLOGY   OF   EPITHELIAL  AND    CONNECTIVE   TISSUES.       35 

Retiform  and  adenoid  tissue  are  also  modifications  of  areolar  tissue. 
The  meshes  of  the  former  contain  but  little  ground  substance,  its  place  being 
taken  by  fluid;  the  meshes  of  the  latter  contain  large  numbers  of  lymph 
corpuscles. 

Fibrous  Tissue. — This  variety  of  connective  tissue  is  widely  distributed 
throughout  the  body.  It  constitutes  almost  entirely  the  ligaments  around 
the  joints,  the  tendons  of  the  muscles,  the  membranes  covering  organs  such 
as  the  heart,  liver,  nervous  system,  bones,  etc.  All  fibrous  tissue,  wherever 
found,  can  be  resolved  into  elementary  bundles,  which  on  microscopic  ex- 
amination are  seen  to  consist  of  delicate,  wavy,  transparent,  homogeneous 
fibers,  which  pursue  an  independent  course,  neither  branching  nor  uniting 
with  adjoining  fibers.  A  small  amount  of  ground  substance  serves  to  hold 
them  together.  Fibrous  tissue  is  tough  and  inextensible,  and  in  consequence 
is  admirably  adapted  to  fulfil  various  mechanical  functions  in  the  body. 
It  is,  however,  quite  pliant,  bending  easily  in  all  directions.  When  boiled, 
fibrous  tissue  yields  gelatin,  a  derivative  of  collagen. 

Elastic  Tissue. — The  fibers  of  elastic  tissue  are  usually  associated  in 
varying  proportions  with  the  white  fibrous  tissue;  but  in  some  structures — 
as  the  ligamentum  nucha?,  the  ligamenta  subflava,  the  middle  coat  of  the 
larger  blood-vessels — the  elastic  fibers  are  almost  the  only  elements  present, 
and  give  to  these  structures  a  distinctly  yellow  appearance.  The  fibers 
throughout  their  course  give  off  many  branches,  which  unite  with  adjoining 
branches  to  form  a  more  or  less  close  network.  As  the  name  implies,  these 
fibers  are  highly  elastic,  and  are  capable  of  being  extended  as  much  as  sixty 
per  cent,  before  breaking. 

Cartilaginous  Tissue. — This  form  of  connective  tissue  differs  from 
the  preceding  varieties  chiefly  in  its  density.  As  a  rule,  it  is  firm  in  con- 
sistency, though  somewhat  elastic.  It  is  opaque,  bluish-white  in  color, 
though  in  thin  sections  translucent.  All  cartilaginous  tissues  consist  of 
connective-tissue  cells  embedded  in  a  solid  ground  substance.  According 
to  the  amount  and  texture  of  the  ground  substance,  three  principal  varieties 
may  be  distinguished: 

1.  Hyaline  cartilage,  in  which  the  cells,  relatively  few  in  number,  are  em- 
bedded in  an  abundant  quantity  of  ground  substance.  The  body  of 
the  cells  is  in  many  instances  distinctly  marked  off  from  the  surround- 
ing substance  by  concentric  fines  or  fibers,  which  form  a  capsule  for  the 
cell.  Repeated  dvision  of  the  cell  substance  takes  place,  until  the  whole 
capsule  is  completely  occupied  by  daughter  cells.  The  ground  substance 
is  pervaded  by  minute  channels,  which  communicate  on  one  hand  with 
the  spaces  around  the  cells,  and  on  the  other  with  lymph-spaces  in  the 


36  HUMAN  PHYSIOLOGY. 

connective  tissue  surrounding  the  cartilage.  By  means  of  these  channels, 
nutritive  fluid  can  permeate  the  entire  structure.  Hyaline  cartilage  is 
found  on  the  ends  of  the  long  bones,  where  it  enters  into  the  formation 
of  the  joints;  between  the  ribs  and  sternum,  forming  the  costal  cartilage, 
as  well  as  in  the  nose  and  larynx. 

2.  White  fibro-cartilage,  the  ground  substance  of  which  is  pervaded  by 
white  fibers,  arranged  in  bundles  or  layers,  between  which  are  scattered 
the  usual  encapsulated  cells.  White  fibro-cartilage  is  tough,  resistant, 
but  flexible,  and  is  found  in  joints  where  strength  and  fixedness  are  re- 
quired. Hence  it  is  present  between  the  vertebrae,  forming  the  inter- 
vertebral discs,  between  the  condyle  of  the  lower  jaw  and  the  glenoid 
fossa,  in  the  knee-joint,  around  the  margin  of  the  joint  cavities,  etc.  In 
these  situations  it  assists  in  maintaining  the  apposition  of  the  bones, 
in  giving  a  certain  degree  of  mobility  to  the  joints,  and  in  diminishing  the 
effects  of  shock  and  pressure  imparted  to  the  bones. 

3.  Yellow  fibro-cartilage,  the  ground  substance  of  which  is  pervaded  by 
opaque,  yellow  elastic  fibers,  which  form,  by  the  interlacing  of  their 
branches,  a  complicated  network,  in  the  meshes  of  which  are  to  be  found 
the  usual  corpuscles.  As  these  fibers  are  elastic,  they  impart  to  the 
cartilage  a  very  considerable  degree  of  elasticity.  Yellow  fibro-cartilage 
is  well  adapted,  therefore,  for  entering  into  the  formation  of  the  external 
ear,  epiglottis,  Eustachian  tube,  etc. — structures  which  require  for  their 
functional  activity  a  certain  degree  of  flexibility  and  elasticity. 

Osseous  Tissue. — Osseous  tissue,  as  distinguished  from  bone,  is  a  member 
of  the  connective-tissue  group,  the  ground  substance  of  which  is  permeated 
with  insoluble  lime  salts,  of  which  the  phosphate  and  carbonate  are  the 
most  abundant.  Immersed  in  dilute  solutions  of  hydrochloric  acid,  they  can 
be  converted  into  soluble  salts  and  dissolved  out.  The  osseous  matrix  left 
behind  is  soft  and  pliable.     When  boiled,  it  yields  gelatin. 

A  thin,  transverse  section  of  a  decalcified  bone,  when  examined  micro- 
scopically, reveals  a  number  of  small,  round  or  oval  openings,  which  repre- 
sent transverse  sections  of  canals  which  run  though  the  bone,  for  the  most 
part  in  a  longitudinal  direction,  though  frequently  anastomosing  with  one 
another.  These  so-called  Haversian  canals  in  the  living  state  contain  blood- 
vessels and  lymphatics. 

Around  each  Haversian  canal  is  a  series  of  concentric  laminae,  composed 
of  white  fibers.  Between  every  two  laminae  are  found  small  cavities  (lacunae), 
from  which  radiate  in  all  directions  small  canals  (canaliculi),  which  com- 
municate freely  with  one  another.  The  Haversian  canals,  with  their  asso- 
ciated lacunae  and  canaliculi,  form  a  system  of  intercommunicating  passage, 


HISTOLOGY   OF    EPITHELIAL  AND    CONNECTIVE   TISSUES.      37 

through  which  lymph  circulates  destined  for  the  nourishment  of  bone. 
Each  lacuna  contains  the  bone  corpuscle,  which  bears  a  close  resemblance  to 
the  usual  branched  connective-tissue  corpuscle,  and  whose  function  appears 
to  be  the  maintenance  of  the  nutrition  of  the  bone. 

The  surface  of  every  bone  in  the  recent  state  is  invested  with  a  fibrous 
membrane,  the  periosteum,  except  where  it  is  covered  with  cartilage.  The 
inner  surface  of  this  membrane  is  loose  in  texture,  and  supports  a  fine  plexus 
of  capillary  blood-vessels  and  numerous  protoplasmic  cells — the  osteoblasts. 
As  this  layer  is  directly  concerned  in  the  formation  of  bone,  it  is  spoken  of  as 
the  osteo genetic  layer. 

A  section  of  a  bone  shows  that  it  is  composed  of  two  kinds  of  tissue — com- 
pact and  cancellated.  The  compact  is  dense,  resembling  ivory,  and  is  found 
on  the  outer  portion  of  the  bone;  the  cancellated  is  spongy,  and  appears  to  be 
made  up  of  thin,  bony  plates,  which  intersect  one  another  in  all  directions, 
and  is  found  in  greatest  abundance  in  the  interior  of  the  bones.  The  shaft 
of  a  long  bone  is  hollow.  This  central  cavity,  which  extends  from  one  end 
of  the  bone  to  the  other,  as  well  as  the  interstices  of  the  cancellated  tissue,  is 
filled  in  the  living  state  with  marrow.  The  marrow  or  medulla  is  composed 
of  a  connective-tissue  framework  supporting  blood-vessels.  In  its  meshes 
are  to  be  found  characteristic  bone  cells  or  osteoblasts,  the  function  of  which 
is  supposed  to  be  the  formation  of  bone.  In  the  long  bones  the  marrow  is 
yellow,  from  the  presence  in  the  connective-tissue  corpuscle  of  fat  lobules, 
which  arise  through  the  transformation  of  the  cell  protoplasm.  In  the  can- 
cellated tissue,  near  the  extremities  of  the  long  bones,  this  fatty  transformation 
does  not  take  place  to  the  same  extent,  and  the  marrow  appears  red.  The 
cells  of  the  red  marrow  are  believed  to  give  birth  indirectly  to  the  red  blood- 
corpuscles. 

Physical    and    Physiologic    Properties    of    Connective    Tissues. — 

Among  the  physical  properties  may  be  mentioned  consistency,  cohesion, 
and  elasticity.  Their  consistency  varies  from  the  semiliquid  to  the  solid 
state,  and  depends  on  the  quantity  of  water  which  enters  into  their  compo- 
position.  Their  cohesion,  except  in  the  softer  varieties,  is  very  considerable, 
and  offers  great  resistence  to  traction,  pressure,  torsion,  etc.  In  all  the  move- 
ments of  the  body,  in  the  contraction  of  muscles,  in  the  performance  of  work, 
the  consistence  and  cohesion  of  these  tissues  play  most  important  roles. 
Wherever  the  various  forms  of  connective  tissue  are  found,  their  chemic 
composition  and  structure  are  in  relation  to  their  functions.  If  traction  be 
the  preponderating  force,  the  structure  becomes  fibrous,  as  in  ligaments  and 
tendons,  and  the  cohesion  greatest  in  the  longitudinal  direction.  If  pressure 
be  exerted  in  all  directions,  as  upon  membranes,  the  fibers  interlace  and 


38  HUMAN  PHYSIOLOGY. 

offer  a  uniform  resistance.  When  pressure  is  exerted  in  a  definite  direction, 
as  on  the  extremities  of  the  long  bones,  the  tissue  becomes  expanded  and 
cancellated.  The  lamellae  of  the  cancellated  tissue  arrange  themselves  in 
curves  which  correspond  to  the  direction  of  the  greatest  pressure  or  traction. 
Extensibility  is  not  a  characteristic  feature,  except  in  those  forms  containing 
an  abundance  of  yellow  elastic  fibers.  The  elasticity  is  an  essential  factor 
in  many  physiologic  actions.  It  not  only  opposes  and  limits  forces  of 
traction,  pressure,  torsion,  etc.,  but  on  their  cessation  returns  the  tissues  or 
organs  to  their  original  condition.  Elasticity  thus  assists  in  maintaining 
the  natural  form  and  position  of  the  organs  by  counterbalancing  and  oppos- 
ing temporarily  acting  forces. 

The  Skeleton. — The  connective  tissues  in  their  entirety  constitute  a 
framework  which  presents  itself  under  two  aspects:  (i)  As  a  solid,  bony 
skeleton,  situated  in  the  trunk  and  limbs,  affording  attachment  for  muscles 
and  viscera;  (2)  as  a  fine,  fibrous  skeleton,  found  everywhere  thoughout 
the  body,  connecting  the  various  viscera  and  affording  support  for  the  epi- 
thelial, muscle,  and  nerve  tissues. 

THE  PHYSIOLOGY  OF  THE  SKELETON. 

The  animal  body  is  characterized  by  the  power  of  executing  a  great  variety 
of  movements,  all  of  which  have  reference  to  a  change  of  relation  of  one 
part  of  the  body  to  another,  or  to  a  change  of  position  of  the  individual  in 
space,  as  in  the  various  acts  of  locomotion.  If  in  the  execution  of  these  move- 
ments the  different  parts  are  applied  or  directed  to  the  overcoming  of  oppos- 
ing forces  in  the  environment,  the  animal  is  said  to  be  doing  work.  In  the 
conception  of  the  animal  body  as  a  machine  for  the  accomplishment  of 
work  the  skeleton,  the  muscle  and  nerve  tissues,  constitute  the  three  primary 
mechanisms,  all  of  which  bear  certain  definite  relations  one  to  another. 

The  Skeleton  is  the  passive  framework,  the  axial  portion  of  which  (the 
vertebral  column,  head,  ribs,  and  sternum)  impart  more  or  less  fixity  and 
rigidity,  while  the  appendicular  portions  (the  bones  of  the  arms  and  legs) 
impart  extreme  mobility.  The  bones  of  the  arms  and  legs  more  especially 
may  be  looked  upon  as  constituting  a  system  of  levers,  the  fulcra  of  which, 
the  points  of  rest  around  which  they  move,  lie  in  the  joints. 

That  a  lever  may  be  effective  as  an  instrument  for  the  accomplishment 
of  work,  it  must  not  only  be  capable  of  moving  around  its  fulcrum,  but  it 
must  at  the  same  time  be  acted  on  by  two  opposing  forces,  one  passive, 
the  other  active.  In  the  movement  of  the  bony  levers  of  the  animal  body, 
the  passive  forces  are  largely  those  connected  with  the  evironment,  e.  g., 


THE   PHYSIOLOGY    OF    THE    SKELETON.  39 

gravity,  cohesion,  friction,  elasticity,  etc.  The  active  forces  by  which  these 
latter  are  opposed  and  overcome  through  the  intermediation  of  the  bony 
levers  are  found  in  the  muscles  attached  to  them.  For  the  execution  of  all 
these  movements,  it  is  essential  that  the  relation  of  the  various  portions  of  the 
bony  skeleton  to  one  another  shall  be  such  as  to  permit  of  movement  while 
yet  retaining  close  apposition.  This  is  accomplished  by  the  mechanical 
conditions  which  have  been  evolved  at  the  points  of  union  of  bones,  and 
which  are  technically  known  as  articulations  or  joints. 

A  consideration  of  the  body  movements  involves  an  account  of  (1)  the  static 
conditions,  or  those  states  of  equilibrium  in  which  the  body  is  at  rest — e.  g., 
standing,  sitting;  (2)  the  dynamic  conditions,  or  those  states  of  activity 
characterized  by  movement — e.  g.,  walking,  running,  etc.  In  this  connection, 
however,  only  those  physical  and  physiologic  peculiarities  of  the  skeleton, 
especially  in  its  relation  to  joints,  will  be  referred  to  which  underlie  and 
determine  both  the  static  and  dynamic  states  of  the  body. 

Structure  of  Joints. — The  structures  entering  into  the  formation  of 
joints  are: 

1.  Bones,  the  articulating  surfaces  of  which  are  often  more  or  less  expanded, 
especially  in  the  case  of  long  bones,  and  at  the  same  time  variously  modi- 
fied and  adapted  to  one  another  in  accordance  with  the  character  and 
extent  of  the  movements  which  there  take  place. 

2.  Hyaline  cartilage,  which  is  closely  applied  to  the  articulating  end  of 
each  bone.  The  smoothness  of  this  form  of  cartilage  facilitates  the 
movements  of  the  opposing  surfaces,  while  its  elasticity  diminishes  the 
force  of  shocks  and  jars  imparted  to  the  bones  during  various  muscular 
acts.  In  a  number  of  joints,  plates  or  discs  of  white  fibro-cartilage  are 
inserted  between  the  surfaces  of  the  bones. 

3.  A  synovial  membrane,  which  is  attached  to  the  edge  of  the  hyaline  cartilage 
entirely  inclosing  the  cavity  of  the  joint.  This  membrane  is  composed 
largely  of  connective  tissue,  the  inner  surface  of  which  is  lined  by  endothelial 
cells,  which  secrete  a  clear,  colorless,  viscid  fluid — the  synovia.  This 
fluid  not  only  fills  up  the  joint-cavity,  but,  flowing  over  the  articulating 
surfaces,  diminishes  or  prevents  friction. 

4.  Ligaments — tough,  inelastic  bands,  composed  of  white  fibrous  tissue — 
which  pass  from  bone  to  bone  in  various  directions  on  the  different  aspects 
of  the  joint.  As  white  fibrous  tissue  is  inextensible  but  pliant,  ligaments 
assist  in  keeping  the  bones  in  apposition,  and  prevent  displacement  while 
yet  permitting  of  free  and  easy  movements. 


40  HUMAN  PHYSIOLOGY. 

Classification  of  Joints. — All  joints  may  be  divided,  according  to  the 
extent  and  kind  of  movements  permitted  by  them,  into  (i)  diarthroses; 
(2)  amphiarthroses;  (3)  synarthroses. 

1.  Diarthroses. — In  this  division  of  the  joints  are  included  all  those  which 
permit  of  free  movement.  In  the  majority  of  instances  the  articulating 
surfaces  are  mutually  adapted  to  each  other.  If  the  articulating  surface 
of  one  bone  is  convex,  the  opposing  but  corresponding  surface  is  concave. 
Each  surface,  therefore,  represents  a  section  of  a  sphere  or  cylinder, 
which  latter  arises  by  rotation  of  a  line  around  an  axis  in  space.  Accord- 
ing to  the  number  of  axes  around  which  the  movements  take  place  all 
diarthrodial  joints  may  be  divided  into: 

1.  Uniaxial  Joints. — In  this  group  the  convex  articulating  surface  is  a 
segment  of  a  cylinder  or  cone,  to  which  the  opposing  surface  more  or 
less  completely  corresponds.  In  such  a  joint  the  single  axis  of  rotation, 
though,  practically  is  not  exactly  at  right  angles  to  the  long  axis  of  the  bone, 
and  hence  the  movements — flexion  and  extension — which  take  place  are 
not  confined  to  one  plane.  Joints  of  this  character — e.  g.,  the  elbow, 
knee,  ankle,  the  phalangeal  joints  of  the  fingers  and  toes — are,  therefore, 
termed  ginglymi,  or  hinge-joints.  Owing  to  the  obliquity  of  their  articulating 
surfaces,  the  elbow  and  ankle  are  cochleoid  or  screw- ginglymi.  Inasmuch 
as  the  axes  of  these  joints  on  the  opposite  sides  of  the  body  are  not  coin- 
cident, the  right  elbow  and  left  ankle  are  right-handed  screws;  the  left 
elbow  and  right  ankle,  left-handed  screws.  In  the  knee-joint  the  form 
and  arrangement  of  the  articulating  surfaces  are  such  as  to  produce  that 
modification  of  a  simple  hinge  known  as  a  spirial  hinge,  or  helicoid.  As 
the  articulating  surfaces  of  the  condyles  of  the  femur  increase  in  convexity 
from  before  backward,  and  as  the  inner  condyle  is  longer  than  the  outer, 
and  therefore,  represents  a  spiral  surface,  the  line  of  translation  or  the  move- 
ment of  the  leg  is  also  a  spiral  movement.  During  flexion  of  the  leg 
there  is  a  simultaneous  inward  rotation  around  a  vertical  axis  passing 
through  the  outer  condyle  of  the  femur;  during  extension  a  reverse  move- 
ment takes  place.  Moreover,  the  slightly  concave  articulating  surfaces 
of  the  tibia  do  not  revolve  around  a  single  fixed  transverse  axis,  as  in  the 
elbow-joint,  for  during  flexion  they  slide  backward,  during  extension 
forward,  around  a  shifting  axis,  which  varies  in  position  with  the  point  of 
contact. 

In  some  few  instances  the  long  axis  of  the  articulating  surface  is  parallel 
rather  than  transverse  to  the  long  axis,  and  as  the  movement  then  takes 
place  around  a  more  or  less  conic  surface,  the  joint  is  termed  a  trochoid 
or  pulley — e.  g.,  the  odonto-atlantal  and  the  radio-ulnar.    In  the  former 


THE   PHYSIOLOGY   OF   THE    SKELETON.  4 1 

the  collar  formed  by  the  atlas  and  its  transverse  ligament  rotates  around 
the  vertical  odontoid  process  of  the  axis.  In  the  latter  the  head  of  the  radius 
revolves  around  its  own  long  axis  upon  the  ulna,  giving  rise  to  the  move- 
ments of  pronation  and  supination  of  the  hand.  The  axis  around  which 
these  two  movements  take  place  is  continued  through  the  head  of  the 
radius  to  the  styloid  process  of  the  ulna. 

Biaxial  Joints. — In  this  group  the  articulating  surfaces  are  unequally 
curved,  though  intersecting  each  other.  When  the  surfaces  lie  in  the  same 
direction,  the  joint  is  termed  an  ovoid  joint — e.  g.,  the  radio-carpal  and 
the  atlanto-occipital.  As  the  axes  of  these  surfaces  are  vertical  to  each 
other,  the  movements  permitted  by  the  former  joint  are  flexion,  extension, 
adduction,  and  abduction,  combined  with  a  slight  amount  of  circumduc- 
tion; the  latter  joint  permits  of  flexion  and  extension  of  the  head,  with 
inclination  to  either  side.  When  the  surfaces  do  not  take  the  same  direc- 
tion, the  joint,  from  its  resemblance  to  the  surfaces  of  a  saddle,  is  termed  a 
saddle-joint — e.  g.,  the  trapezio-metacarpal.  The  movements  permitted 
by  this  joint  are  also  flexion,  extension,  adduction,  abduction,  and 
circumduction. 

Polyaxial  Joints. — In  this  group  the  convex  articulating  surface  is  a 
segment  of  a  sphere,  which  is  received  by  a  socket  formed  by  the  opposing 
articulating  surface.  In  such  a  joint,  termed  an  enarthrodial  or  ball-and- 
socket  joint — e.  g.,  the  shoulder-joint,  hip-joint — the  distal  bone  revolves 
around  an  indefinite  number  of  axes,  all  of  which  intersect  one  another  at 
the  center  of  rotation.  For  simplicity,  however,  the  movement  may  be 
described  as  taking  place  around  axes  in  the  three  ordinal  planes — viz.,  a 
transverse,  a  sagittal,  and  a  vertical  axis.  The  movements  around  the 
transverse  axis  are  termed  flexion  and  extension;  around  the  sagittal  axis, 
adduction  and  abduction;  around  the  vertical  axis,  rotation.  When  the 
bone  revolve  around  the  surface  of  an  imaginary  cone,  the  apex  of  which 
is  the  center  of  rotation  and  the  base  the  curve  described  by  the  hand,  the 
movement  is  termed  circumduction. 

.  Amphiarthroses. — In  this  division  are  included  all  those  joints  which 
permit  of  but  slight  movement — e.  g.,  the  intervertebral,  the  interpubic, 
and  the  sacro-iliac  joints.  The  surfaces  of  the  opposing  bones  are  united 
and  held  in  position  largely  by  the  intervention  of  a  firm,  elastic  disc  of 
fibro-cartilage.     Each  joint  is  also  strengthened  by  ligaments. 

.  Synarthroses. — In  this  division  are  included  all  those  joints  in  which 

the  opposing  surfaces  of  the  bones  are  immovably  united,  and  hence 

do  not  permit  of  any  movement — e.  g.,  the  joints  between  the  bones 
of  the  skull. 


42  HUMAN  PHYSIOLOGY. 

The  Vertebral  Column. — In  all  static  and  dynamic  states  of  the  body  the 
vertebral  column  plays  a  most  essential  role.  Situated  in  the  middle  of  the 
back  of  the  trunk,  it  forms  the  foundation  of  the  entire  skeleton.  It  is  com- 
posed of  a  series  of  superimposed  bones,  termed  vertebrae,  which  increase  in 
size  from  above  downward  as  far  as  the  brim  of  the  pelvic  cavity.  Supe- 
riorly, it  supports  the  skull;  laterally,  it  affords  attachment  for  the  ribs,  which 
in  turn  support  the  weight  of  the  upper  extremities;  below,  it  rests  upon  the 
pelvic  bones,  which  transmit  the  weight  of  the  body  to  the  inferior  extremities. 
The  bodies  of  the  vertebras  are  united  one  to  another  by  tough  elastic  discs 
of  fibro-cartilage,  which,  collectively,  constitute  about  one-quarter  of  the 
length  of  the  vertebral  column.  The  vertebrae  are  held  together  by  liga- 
ments situated  on  the  anterior  and  posterior  surfaces  of  their  bodies,  and  by 
short,  elastic  ligaments  between  the  neural  arches  and  processes.  These 
structures  combine  to  render  the  vertebral  column  elastic  and  flexible,  and 
enable  it  to  resist  and  diminish  the  force  of  shocks  communicated  to  it. 

The  amphiarthrodial  character  of  the  intervertebral  joint  endows  the  entire 
column  with  certain  forms  of  movement  which  are  necessary  to  the  perform- 
ance of  many  body  activities.  While  the  range  of  movement  between  any 
two  vertebrae  is  slight,  the  sum  total  of  movement  of  the  entire  series  of 
vertebrae  is  considerable.  In  different  regions  of  the  column  the  character, 
as  well  as  the  range  of  movement,  varies  in  accordance  with  the  form  of  the 
vertebrae  and  the  inclination  of  their  articular  processes.  In  the  cervical  and 
lumbar  regions  extension  and  flexion  are  freely  permitted,  though  the  former 
is  greater  in  the  cervical,  the  latter  in  the  lumbar  region,  especially  between 
the  fourth  and  fifth  vertebrae.  Lateral  flexion  takes  place  in  all  portions  of 
the  column,  but  is  particularly  marked  in  the  cervical  region.  A  rotatory 
movement  of  the  column  as  a  whole  takes  place  through  an  angle  of  about 
twenty-eight  degrees.  This  is  most  evident  in  the  lower  cervical  and  dorsal 
regions. 

The  skeleton  may,  therefore  be  regarded  as  a  highly  developed  frame- 
work, which  determines  not  only  the  form  of  the  body,  and  affords  support 
and  protection  to  the  various  softer  organs  and  tissues,  but  also,  through  the 
mobility  of  its  joints,  permits  of  a  great  variety  of  complicated  movements. 

GENERAL  PHYSIOLOGY   OF  MUSCLE  TISSUE. 

The  muscle  tissue,  which  closely  invests  the  bones  of  the  body,  and 
which  is  familiar  to  all  as  the  flesh  of  animals,  is  the  immediate  cause  of  the 
active  movements  of  the  body.  This  tissue  is  grouped  in  masses  of  varying 
size  and  shape,  which  are  technically  known  as  muscles.  The  majority 
of  the  muscles  of  the  body  are  connected  with  the  bones  of  the  skeleton  in 


GENERAL   PHYSIOLOGY   OF   MUSCULAR   TISSUE.  43 

such  a  manner  that,  by  an  alteration  in  their  form,  they  can  change  not  only 
the  position  of  the  bones  with  reference  to  one  another,  but  can  also  change 
the  individual's  relation  to  surrounding  objects.  They  are,  therefore,  the 
active  organs  of  both  motion  and  locomotion,  in  contradistinction  to  the 
bones  and  joints,  which  are  but  passive  agents  in  the  performance  of  the 
corresponding  movements.  In  addition  to  the  muscle  masses  which  are 
attached  to  the  skeleton,  there  are  also  other  collections  of  muscle  tissue  sur- 
rounding cavities  such  as  the  stomach,  intestine,  blood-vessels,  etc.,  which 
impart  to  their  walls  motility,  and  so  influence  the  passage  of  a  material 
through  them. 

Muscles  produce  movement  of  the  structures  to  which  they  are  attached 
by  the  property  with  which  they  are  endowed  of  changing  their  shape,  short- 
ening or  contracting  under  the  influence  of  a  stimulus  transmitted  to  them 
from  the  nervous  system.     Muscles  are  therefore  divided  into: 

1.  Voluntary  muscles,  comprising  those  whose  activity  is  called  forth  by 
stimuli  of  the  nerves  as  the  result  of  an  act  or  effort  of  volition. 

2.  Involuntary  muscles,  comprising  those  whose  activity  is  entirely  inde- 
pendent of  the  volition. 

The  voluntary  muscles  are  also  known  from  their  attachment  to  the 
skeleton  as  skeletal,  and  from  their  microscopic  appearance  as  striped  muscles. 
The  involuntary  muscles,  from  their  relation  to  the  viscera  of  the  body,  are 
known  also  as  visceral,  and  from  their  microscopic  appearance  as  plain  or 
smooth  muscles. 

General  Structure  of  Muscles. — All  skeletal  muscles  consist  of  a  central 
fleshy  portion,  the  body  or  belly,  which  is  provided  at  either  extremity  with 
a  tendon  in  the  form  of  a  cord  or  membrane  by  which  it  is  attached  to  the 
bones.  The  body  is  the  contractile  region,  the  source  of  activity;  the  tendon 
is  a  passive  region,  and  merejy  transmits  the  activity  to  the  bones. 

A  skeletal  muscle  is  a  complex  organ  consisting  of  muscular  fibers,  con- 
nective tissue,  blood-vessels,  and  lymphatics.  The  general  body  of  the 
muscle  is  surrounded  by  a  dense  layer  of  connective  tissue,  the  epimysium, 
which  blends  with  and  partly  forms  the  tendon;  from  its  inner  surface  septa 
of  connective  tissue  pass  inward  and  group  the  muscle-fibers  into  larger  and 
smaller  bundles,  termed  fasciculi.  The  fasciculi,  invested  by  this  special 
sheath,  the  perimysium,  are  irregular  in  shape,  and  vary  considerably  in 
size.  The  fibers  of  the  fasciculi  are  separated  from  one  another  and  sup- 
ported by  a  delicate  connective  tissue,  the  endomysium.  The  connective 
tissue  thus  surrounding  and  penetrating  the  muscle  binds  its  fibers  into  a 
distinct  organ,  and  affords  support  to  blood-vessels,  nerves,  and  lymphatics. 
The  muscle  fibers  are  arranged  parallel  to  one  another,  and  their  direction 


44  HUMAN  PHYSIOLOGY. 

is  that  of  the  long  axis  of  the  muscle.     In  length  they  vary  from  thirty  to 
forty  millimeters,  and  in  diameter  from  twenty  to  thirty  micromillimeters. 

The  vascular  supply  to  the  muscle  is  very  great,  and  the  disposition  of 
the  capillary  vessels,  with  reference  to  muscle-fiber,  is  very  characteristic 
The  arterial  vessels,  after  entering  the  muscle,  are  supported  by  the  perim- 
ysium; in  this  situation  they  give  off  short,  transverse  branches,  which  imme- 
diately break  up  into  a  capillary  network  of  rectangular  shape,  within  which 
the  muscle-fibers  are  contained.  The  muscle-fiber  in  intimate  relation  with 
the  capillary  is  bathed  with  lymph  derived  from  it.  Its  contractile  substance, 
however,  is  separated  from  the  lymph  by  its  own  investing  membrane, 
through  which  all  interchange  of  nutritive  and  waste  materials  must  take 
place.  Lymphatics  are  present  in  muscle,  but  are  confined  to  the  con- 
nective tissue,  in  the  spaces  of  which  they  have  their  origin. 

The  nerves  which  carry  the  stimuli  to  a  muscle  enter  near  its  geometric 
center.  Many  of  the  fibers  pass  directly  to  the  muscle-fibers  with  which  they 
are  connected;  others  are  distributed  to  blood-vessels.  Every  muscle-fiber 
is  supplied  with  a  special  nerve-fiber,  except  in  those  instances  where  the 
nerve  trunks  entering  a  muscle  do  not  contain  so  many  fibers  as  the  muscle. 
In  such  cases  the  nerve-fibers  divide,  until  the  number  of  branches  equals 
the  number  of  muscle-fibers.  The  individual  muscle-fiber  is  penetrated  near 
its  center  by  the  nerve,  the  ends  being  practically  free  from  nerve  influence. 
The  stimulus  that  comes  to  the  muscle  fiber  acts  primarily  upon  its  center, 
and  then  travels  in  both  directions  to  the  ends. 

Histology  of  the  Skeletal  Muscle-Fiber. — A  muscle-fiber  consists 
of  a  transparent  elastic  membrane,  the  sar oolemma,  in  which  is  contained  the 
true  muscle  element.  Examined  microscopically,  the  fiber  presents  a  series 
of  alternate  dim  and  bright  bands,  giving  to  it  a  striated  appearance. 

When  the  bright  band  is  examined  with  high  magnifying  powers,  a  fine, 
dark  line  is  seen  crossing  it  transversely.  It  was  supposed  by  Krause  to  be 
the  optic  expression  of  a  membrane  attached  laterally  to  the  sarcolemma. 

The  muscle-fiber  also  exhibits  a  longitudinal  striation,  indicating  that  it 
is  composed  of  fibrillae,  placed  side  by  side  and  embedded  in  some  inter- 
fibrillar  substance,  to  which  the  name  sarcoplasm  has  been  given.  The 
fibrillae,  which  are  arranged  longitudinally  to  the  long  axis  of  the  fiber,  are 
grouped  by  the  intervening  material  into  bundles  of  varying  size,  the  muscle 
columns.  The  fibrillae  which  extend  throughout  the  length  of  the  fiber  are 
apparently  of  uniform  thickness  passing  directly  through  the  transverse 
membrane  and  being  supported  by  it. 

In  the  region  of  the  dim  band  the  fibrilla  presents  itself  in  the  form  of  a 


GENERAL   PHYSIOLOGY    OF   MUSCULAR   TISSUE.  45 

homogeneous  prismatic  rod,  termed  sarcostyle,  separated  from  neighboring 
rods  by  a  slight  amount  of  sarcoplasm. 

Briicke  has  shown  that  when  the  muscle-fiber  is  examined  under  crossed 
Nicol  prisms  the  dim  band  appears  bright  and  the  bright  band  appears 
dim  against  a  dark  background,  indicating  that  the  former  is  doubly  refract- 
ile,  or  anisotropic,  the  latter  singly  refractile,  or  isotropic.  The  fiber,  there- 
fore, appears  to  be  composed  of  alternate  discs  of  anisotropic  and  isotropic 
substance. 

Structure  of  Non-striated  Muscle-fiber. — As  the  name  implies,  the 
involuntary  fiber  is  non-striated,  being  apparently  uniform  and  homogeneous 
in  appearance.  When  isolated,  the  fiber  presents  itself  in  the  form  of  an 
elongated  fusiform  cell,  varying  from  y1^  to  -^  of  an  inch  in  length.  In 
some  animals  the  fiber  exhibits  a  longitudinal  striation,  as  if  it  were  composed 
of  fibers.  The  cell  is  surrounded  by  a  thin,  elastic  membrane,  and  contains 
a  distinct  oval  nucleus.  The  fibers  are  usually  arranged  in  bundles  and 
lamellae,  and  held  together  by  a  cement  substance  and  connective  tissue. 
This  non-striated  muscle  tissue  is  found  in  the  muscularis  mucosas  of  the 
alimentary  canal  as  well  as  in  the  muscular  walls  of  the  stomach  and  intestines 
in  the  posterior  part  of  the  trachea,  in  the  bronchial  tubes,  in  the  walls  of  the 
blood-vessels,  and  in  many  other  situations. 

Chemic  Composition  of  Muscle. — The  chemic  composition  of  muscle  is 
imperfectly  understood,  owing  to  the  fact  that  some  of  its  constituents  undergo 
a  spontaneous  coagulation  after  death,  and  that  the  chemic  methods  em- 
ployed also  tend  to  alter  its  normal  composition.  When  fresh  muscle  is 
freed  from  fat  and  connective  tissue,  frozen,  rubbed  up  in  a  mortar,  and 
expressed  through  linen,  a  slightly  yellow,  syrupy,  alkaline,  or  neutral  fluid 
is  obtained,  known  as  muscle  plasma.  This  fluid  at  normal  temperature 
coagulates  spontaneously,  and  resembles  in  many  respects  the  coagulation  of 
blood  plasma.  The  coagulum  subsequently  contracts  and  squeezes  out 
an  acid  muscle  serum.  The  coagulated  mass  is  termed  myosin  or  myogen 
fibrin.  This  protein  belongs  to  the  class  of  globulins.  Inasmuch  as  it  is  not 
present  in  living  muscle,  and  makes  its  appearance  only  in  the  as  yet  living 
muscle  plasma,  it  is  probable  that  it  is  derived  from  some  preexisting  sub- 
stance, which  is  supposed  to  be  myosinogen  or  myogen.  Myosin  is  digested 
by  pepsin  and  trypsin.  According  to  Halliburton,  muscle  plasma  contains 
the  following  protein  bodies:  Myosinogen,  paramyosinogen,  albumin, 
myoalbumose,  all  of  which  differ  in  chemic  composition  and  respond  to 
various  chemic  and  physical  reagents. 

Ferment  bodies,  such  as  pepsin  and  diastase;  non-nitrogenized  bodies, 
such  as  glycogen,  lactic  and  sarcolactic  acids,  fatty  bodies,  and  inosite; 


46  HUMAN  PHYSIOLOGY. 

nitrogenized  extractives — e.  g.,  urea,  uric  acid,  kreatinin,  as  well  as  inorganic 
salts,  have  been  obtained  from  the  muscle  serum. 

Metabolism  in  Muscles. — The  chemic  changes  which  underlie  the  trans 
formation  of  energy  in  living  muscles  are  very  active  and  complex. 

As  shown  by  an  analysis  of  the  blood  flowing  to  and  from  the  resting 
muscle,  it  has,  while  passing  through  the  capillaries,  lost  oxygen  and  gained 
carbon  dioxid.  The  amount  of  oxygen  absorbed  by  the  muscle  (nine  per 
cent.)  is  greater  than  the  amount  of  C02  given  off  (6.7  per  cent.).  There  is 
no  parallelism  between  these  two  processes,  as  C02  will  be  given  off  in  the 
absence  of  oxygen,  or  in  an  atmosphere  of  nitrogen. 

In  the  active  or  contracting  muscle  both  the  absorption  of  oxygen  and  the 
production  of  C02  are  largely  increased,  but  the  ratio  existing  between  them 
differs  considerably  from  that  of  the  resting  muscle,  for  the  quantity  of  oxygen 
absorbed  amounts  to  11.26  per  cent.,  the  quantity  of  C02  to  10.8  per  cent. 
(Ludwig).  Moreover,  in  a  tetanized  muscle  the  quantity  of  C02  given  off 
may  be  largely  in  excess  of  the  oxygen  absorbed.  From  these  facts  it  is 
evident  that  the  energy  of  the  contraction  does  not  depend  upon  the  direct 
oxidation  of  certain  substances,  but  upon  the  decomposition  of  some  unstable 
compound  of  high  potential  energy,  rich  in  carbon  and  oxygen.  When  the 
muscle  is  active,  its  tissue  changes  from  a  neutral  to  an  acid  reaction,  from 
the  development  of  sarcolactic  and  possibly  phosphoric  acids.  The  amount 
of  glycogen  present  in  muscle  (0.43  per  cent.)  diminishes,  but  muscles  want- 
ing in  glycogen,  nevertheless,  retain  their  power  of  contraction.  Water  is 
absorbed.  The  amount  of  urea  is  not  materially  increased  by  muscular 
activity,  unless  it  is  excessive  and  prolonged,  and  then  only  in  the  absence  of 
a  sufficient  quantity  of  non-nitrogenized  material.  Coincident  with 
muscle  contraction,  the  blood-vessels  become  widely  dilated,  leading  to 
a  large  increase  in  the  blood-supply  and  a  rapid  removal  of  products  of 
decomposition. 

Rigor  Mortis. — A  short  time  after  death  the  muscles  pass  into  a  condition 
of  extreme  rigidity  or  contraction,  which  lasts  from  one  to  five  days.  In 
this  state  they  offer  great  resistance  to  extension,  their  tonicity  disappears, 
their  cohesion  diminishes,  their  irritability  ceases.  The  time  of  the  appear- 
ance of  this  post-mortem  or  cadaveric  rigidity  varies  from  a  quarter  of  an 
hour  to  seven  hours.  Its  onset  and  duration  are  influenced  by  the  condition 
of  the  muscular  irritability  at  the  time  of  death.  When  the  irritability  is 
impaired  from  any  cause,  such  as  disease  or  defective  blood-supply,  the  rigid- 
ity appears  promptly,  but  is  of  short  duration.  After  death  from  acute 
diseases,  it  is  apt  to  be  delayed,  but  to  continue  for  a  longer  period. 

The  rigidity  appears  first  in  the  muscles  of  the  lower  jaw  and  neck; next 


GENERAL   PHYSIOLOGY    OF   MUSCULAR   TISSUE.  47 

in  the  muscles  of  the  abdomen  and  upper  extremities;  finally  in  the  trunk  and 
lower  extremities.     It  disappears  in  practically  the  same  order. 

Chemic  changes  of  a  marked  character  accompany  this  rigidity.  The 
muscle  becomes  acid  in  reaction  from  the  development  of  sarcolactic  acid ; 
it  gives  off  a  large  quantity  of  carbonic  acid,  and  is  shortened  and  dimin- 
ished in  volume. 

The  immediate  cause  of  the  rigidity  appears  to  be  a  coagulation  of  the 
myosinogen  within  the  sarcolemma,  with  the  subsequent  formation  of  myosin 
and  muscle  serum.  In  the  early  stages  of  coagulation  restitution  is  possible 
by  the  circulation  of  arterial  blood  through  the  vessels.  The  final  disap- 
pearance of  this  contraction  is  due  to  the  action  of  acids  dissolving  the  myosin, 
and  possibly  to  putrefactive  changes. 

Source  of  Muscle  Energy. — According  to  most  experimenters,  it  is  cer- 
tain that  normal  muscle  activity  is  not  dependent  on  the  metabolism  of  nitrog- 
enous materials,  inasmuch  as  its  chief  end  product,  urea,  is  not  increased. 
The  marked  production  of  C02  points  to  the  decomposition  of  some  unstable 
compound  of  a  carbohydrate  character,  rich  in  carbon  and  hydrogen.  It 
has  been  suggested  that  glycogen  furnishes  the  energy  after  it  has  been 
transformed  into  sugar.  Muscles  wanting  in  glycogen  are,  nevertheless, 
capable  of  contracting  for  some  time.  It  has  been  suggested  by  Hermann 
that  the  energy  of  a  muscular  contraction  may  be  due  to  the  splitting  and 
subsequent  re-formation  of  a  complex  body  belonging  neither  to  the  carbo- 
hydrates nor  to  the  fats,  but  to  the  albumins.  To  this  body  the  term  inogen 
has  been  applied.  This  complex  molecule,  the  product  of  the  metabolic 
activity  of  the  muscle  cell,  in  undergoing  decomposition  would  yield  C02 
sarcolactic  acid,  and  a  protein  residue  resembling  myosin.  With  the 
cessation  of  the  contraction,  the  muscle  protoplasm  recombines  the 
protein  residue  with  oxygen,  carbohydrates,  and  fats,  and  again  forms 
inogen. 

The  phenomena  of  rigor  mortis  support  such  a  view.  At  the  moment 
of  this  contraction  the  muscle  gives  off  C02  in  large  amount,  the  muscle 
becomes  acid,  and  myosin  is  formed.  There  is  thus  a  close  analogy  between 
the  two  processes;  in  other  words,  a  contraction  is  a  partial  death  of  the  muscle. 
As  to  what  becomes  of  the  myosin  formed  during  a  contraction,  nothing  is 
known.     It  may  be  used  in  the  formation  of  new  inogen. 

The  Physical  Properties  of  Muscle  Tissue. — The  consistency  of  muscle 
tissue  varies  considerably,  according  to  the  different  states  of  the  muscle. 
In  a  state  of  tension  it  is  hard  and  resistant;  when  free  from  tension,  it  is 
soft  and  fluctuating,  whether  the  muscle  is  contracting  or  resting.     Tension 


48  HUMAN  PHYSIOLOGY. 

alone  produces  hardness.  The  cohesion  of  muscle  tissue  is  less  than  that 
of  connective  tissue,  and  is  broken  more  readily.  Cohesion  resists  traction 
and  pressure,  and  lasts  as  long  as  irritability  remains. 

The  elasticity  of  a  muscle,  though  not  great  is  almost  perfect.  After 
being  extended  by  a  weight,  it  returns  to  its  natural  form.  The  limit 
of  elasticity,  however,  is  soon  passed.  A  weight  of  50  or  100  grams 
will  overcome  the  elasticity  so  that  it  will  not  return  to  its  natural  length. 
In  inorganic  bodies  the  extension  is  directly  proportional  to  the  extending 
weight,  and  the  line  of  extension  is  straight.  With  muscles,  the  extension 
is  not  proportional  to  the  weight.  While  at  first  it  is  marked,  the  elongation 
diminishes  as  the  weight  increases  by  equal  increments,  so  that  the  line  of 
extension  becomes  a  curve.  In  other  words,  the  elasticity  of  a  passive 
muscle  augments  with  increased  extension.  On  the  contrary  the  elasticity 
of  an  active  is  less  than  that  of  a  passive  muscle,  for  it  is  elongated  more 
by  the  same  weight,  as  shown  by  experiment. 

Tonicity  is  a  property  of  all  muscles  in  the  body,  in  consequence  of  being 
normally  stretched  to  a  slight  extent  beyond  their  natural  length.  This 
may  be  due  to  the  action  of  antagonistic  muscles,  or  to  the  elasticity  of  the 
parts  of  the  skeleton  to  which  they  are  attached.  This  is  shown  by  the 
shortening  of  the  muscle  which  takes  place  when  it  is  divided.  Muscular 
tonus  plays  an  important  rdle  in  muscular  contraction.  Being  always  on  the 
stretch,  the  muscle  loses  no  time  in  acquiring  that  degree  of  tension  necessary 
to  its  immediate  action  on  the  bones.  Again,  the  working  power  of  a  muscle 
is  increased  by  the  presence  of  some  resistance  to  the  act  of  contraction. 
According  to  Marey,  the  amount  of  work  is  considerably  increased  when  the 
muscular  energy  is  transmitted  by  an  elastic  body  to  the  mass  to  be  moved, 
while  at  the  same  time,  the  shock  of  the  contraction  is  lessened.  The  position 
of  a  passive  limb  is  the  resultant  also  of  the  elastic  tension  of  antagonistic 
groups  of  muscles. 

Muscle  excitability  and  contractility  are  terms  employed  to  denote  that 
property  of  muscle  tissue  in  virtue  of  which  it  contracts  or  shortens  in  response 
to  various  excitants  or  stimuli.  Though  usually  associated  with  the  activity 
of  the  nervous  system,  it  is  nevertheless,  an  independent  endowment,  and 
persists  after  all  nervous  connections  are  destroyed.  If  the  nerve  terminals 
be  destroyed,  as  they  can  be  by  the  introduction  of  curara  into  the  system, 
the  muscles  become  completely  relaxed  and  quiescent.  The  strongest  stimuli 
applied  to  the  nerves  fail  to  produce  a  contraction.  Various  external  stimuli 
applied  directly  to  the  muscle  substance  produce  at  once  the  characteristic 
contraction.  The  excitability  of  muscle  is  therefore  an  inherent  property, 
dependent  on  its  nutrition,  and  persisting  as  long  as  it  is  supplied  with  proper 


GENERAL   PHYSIOLOGY    OF   MUSCULAR   TISSUE.  49 

nutritive  materials  and  surrounded  by  those  external  conditions  which  main- 
tain its  chemic  or  physical  integrity. 

Muscle  Contractions. — All  muscle  contractions  occurring  in  the  body 
under  normal  physiologic  conditions  are  either  voluntary,  caused  by  a 
volitional  effort  and  the  transmission  of  a  nerve  impulse  from  the  brain  through 
the  spinal  cord  and  nerves  to  the  muscles,  or  reflex,  caused  by  a  peripheral 
stimulation  and  the  transmission  of  a  nerve  impulse  to  the  spinal  cord,  to  be 
reflected  outward  through  the  same  nerves  to  the  muscles.  In  either  case 
the  resulting  contraction  is  essentially  the  same.  The  normal  or  physiologic 
stimulus  which  provokes  the  musuclar  contraction  is  a  nerve  impulse  the  nature 
of  which  is  unknown,  but  is  perhaps  allied  to  a  molecular  disturbance.  After 
removal  from  the  body,  muscles  remain  in  a  state  of  rest,  inasmuch  as  they 
possess  no  spontaneity  of  action.  Though  consisting  of  a  highly  irritable 
tissue,  they  cannot  pass  from  the  passive  to  the  active  state  except  upon  the 
application  of  some  form  of  stimulation. 

The  stimuli  which  are  capable  of  calling  forth  a  contraction  may  be  divided 
into — 

1.  Mechanical. 

2.  Chemic. 

3.  Physical. 

4.  Electric. 

Every  mechanical  stimulus  of  a  muscle — e.  g.,  pick,  cut,  or  tap — provided 
it  has  sufficient  intensity,  and  is  repeated  with  sufficient  rapidity,  will  cause 
not  only  a  single  contraction,  but  a  series  of  contractions. 

All  chemic  agents  which  impair  the  chemic  composition  of  the  muscle  with 
sufficient  rapidity — e.  g.,  hydrochloric  acid,  acetic  and  oxalic  acids,  distilled 
water  injected  into  the  vessels,  etc. — act  as  stimuli,  and  produce  single  and 
multiple  contractions.  Physical  agents,  as  heat  and  electricity,  also  act  as 
stimuli.  A  muscle  heated  rapidly  to  300  C.  contracts  vigorously,  and 
reaches  its  maximum  at  450  C.  Of  all  forms  of  stimuli,  the  electric  is  the 
most  generally  used.  Two  forms  are  used — the  induced  current  and  the 
make-and-break  of  a  constant  current. 

Changes  in  a  Muscle  During  Contraction. — When  a  muscle  is  stimu- 
lated, either  indirectly  through  the  nerve  or  directly  by  any  external  agent,  it 
undergoes  a  series  of  changes,  which  relate  to  its  form,  volume,  optic,  physical 
chemic,  and  electric  properties.  These  changes,  in  their  totality,  constitute 
the  muscular  contraction. 

1.  Form. — The  most  obvious   change  is  that  of  form.     The  fibers  become 
shorter  in  their  longitudinal  and  wider  in  their  transverse  diameters,  and 

4 


50  HUMAN  PHYSIOLOGY. 

the  muscle  as  a  whole  becomes  shorter  and  thicker.     The  degree  of  short- 
ening may  amount  to  thirty  per  cent,  of  the  original  length. 

2.  Volume. — The  increase  in  transverse  diameter  does  not  fully  compensate 
for  the  dimunition  in  length,  for  there  is  at  the  moment  of  contraction  a 
slight  shrinkage  in  volume,  which  has  been  attributed  to  a  compression  of 
air  in  its  interstices. 

3.  Optic  Changes. — If  a  muscle-fiber  be  examined  microscopically  during  its 
contraction,  it  will  be  observed  that  when  the  contraction  wave  begins, 
both  bright  and  dim  bands  diminish  in  height  and  become  broader,  though 
this  change  is  more  noticeable  in  the  region  of  the  bright  band.  This 
Englemann  attributes  to  a  passage  of  fluid  material  from  the  bright  into 
the  dim  band.  At  the  time  of  relaxation  there  is  a  return  of  this  material, 
and  the  fiber  assumes  its  orignal  shape  and  volume.  As  the  contraction 
wave  reaches  its  maximum,  the  optic  properties  of  both  the  isotropic  and 
anisotropic  bands  change.  The  former,  which  was  originally  clear,  now 
becomes  darker  and  less  transparent,  until  at  the  crest  of  the  wave  it  as- 
sumes the  appearance  of  a  distinct  dark  band.  The  latter,  the  anisotropic, 
which  was  originally  dim,  now  becomes,  in  comparison,  clear  and  light. 
This  change  in  optic  appearance  is  due  to  an  increase  in  refrangibility  of  the 
isotropic  and  a  decrease  in  the  anisotropic  bands  coincident  with  the  passage 
of  fluid  from  the  former  into  the  latter.  There  is  at  the  height  of  the 
contraction  a  complete  reversal  in  the  positions  of  the  striations.  At  a 
certain  stage  between  the  beginning  and  the  crest  of  the  wave  there  is  an 
intermediate  point,  at  which  the  stria?  almost  entirely  disappear,  giving  to 
the  fiber  an  appearance  of  homogeneity.  There  is,  however,  no  change  in 
refractive  power,  as  shown  by  the  polarizing  apparatus.  After  the  con- 
traction wave  has  reached  the  stage  of  greatest  intensity,  there  is  a  reversal 
of  the  foregoing  phenomena,  and  the  fiber  returns  to  its  original  condition, 
which  is  one  of  relaxation. 

Physical  Changes. — The  extensibility  of  muscle  is  increased  during  the  con- 
tracton,  the  same  weight  elongating  the  fibers  to  a  greater  extent  than  during 
rest.  The  elasticity,  or  its  power  of  returning  to  its  original  form  is  corres- 
pondingly diminished. 

Chemic  Changes. — The  metabolism  of  muscle  during  the  contraction  is 
very  active.  There  is  an  increase  in  the  production  of  carbon  dioxid  and  in 
the  absorption  of  oxygen.  The  muscle  changes  from  an  alkaline  or  neutral 
to  an  acid  reaction,  from  the  development  of  sarcolactic  acid.  The  muscle 
also  becomes  warmer.  The  electric  changes  will  be  treated  of  in  connection 
with  nerves. 


GENERAL   PHYSIOLOGY    OF    MUSCULAR    TISSUE.  5 1 

Transmission  of  the  Contraction  Wave. — Normally,  when  a  muscle  is 
stimulated  by  the  nerve  impulse,  the  shortening  and  thickening  of  the  fibers 
begin  at  the  end  organ  and  travel  in  opposite  directions  to  the  ends  of  the 
muscle.  This  change  propagates  itself  in  a  wave-like  manner,  and  has  been 
termed  the  contraction  wave.  If  a  stimulus  be  applied  directly  to  the  end  of 
a  long  muscle,  the  contraction  wave  passes  along  its  entire  length  to  the 
opposite  extremity,  in  virtue  of  the  conductivity  of  muscular  tissue.  The 
rapidity  of  the  propagation  varies  in  different  animals — in  the  frog,  from 
three  to  four  meters  a  second,  in  man,  from  ten  to  thirteen  meters.  The 
length  of  the  wave  varies  from  200  to  400  millimeters. 

Graphic  Record  of  a  Muscle  Contraction. — The  changes  in  the  form  of 
a  muscle  during  contraction  and  relaxation  have  been  carefully  studied  by 
recording  the  muscle  movement  by  means  of  an  attached  lever,  the  end  of 
which  is  allowed  to  rest  upon  a  moving  surface.  The  time  relations  of  all 
phases  of  the  muscular  movement  are  obtained  by  placing  beneath  the  lever 
a  pen  attached  to  an  electro-magnet  thrown  into  action  by  a  tuning-fork 
vibrating  in  hundredths  of  a  second.  A  marking  lever  records  simultaneously 
the  moment  of  stimulation. 

Single  Contraction. — When  a  single  electric  induction  shock  is  applied 
to  a  nerve  close  to  the  muscle,  the  latter  undergoes  a  quick  pulsation,  speedily 
returning  to  its  former  condition.     As  shown  by  the  muscle  curve  (see  Fig.  4) 


Fig.  4. — Muscle   Curve  Produced   by  a  Single  Induction  Shock  Applied  to 

a  Muscle. — (Landois.) 

a-f.  Abscissa,     a-c.  Ordinate,     a-b.  Period  of  latent  stimulation,     b-d.  Period  o 
increasing  energy,     d-e.  Period  of  decreasing  energy,     e-f.  Elastic  after-vibrations 

there  is  between  the  moment  of  stimulation  and  the  beginning  of  the  contrac- 
tion a  short  but  measurable  period,  known  as  the  latent  period,  during  which 
certain  chemic  changes  are  taking  place  preparatory  to  the  exhibition  of  the 
muscle  movement.  Even  when  the  electric  stimulus  is  applied  directly  to 
the  muscle,  a  latent  period,  though  shorter,  is  observable.  The  duration  of 
this  period  in  the  skeletal  muscles  of  the  frog  has  been  estimated  at  0.01  of 
a  second;  but  it  has  been  shown  by  the  employment  of  more  accurate  methods 
and  the  elimination  of  various  external  influences  to  be  much  less — not  more 
than  0.0033  to  0.0025  °f  a  second. 


52  HUMAN  PHYSIOLOGY. 

The  contraction  follows  the  latent  period.  This  begins  slowly,  rapidly 
reaches  its  maximum,  and  ceases.  This  has  been  termed  the  stage  of  rising 
or  increasing  energy.  The  time  occupied  in  the  stage  of  shortening  is  about 
0.04  of  a  second,  though  this  will  depend  on  the  strength  of  the  stimulus, 
the  load  with  which  the  muscle  is  weighted,  and  the  condition  of  the  muscle 
irritability. 

The  relaxation  immediately  follows  the  contraction.  This  takes  place  at 
first  slowly,  after  which  the  muscle  rapidly  returns  to  its  original  length. 
This  is  the  period  of  falling  or  decreasing  energy,  and  occupies  about  0.05 
of  a  second.  The  whole  duration  of  a  muscle  contraction  occupies,  therefore, 
about  0.1  of  a  second. 

Residual  or  after-vibrations  are  frequently  seen  which  are  due  to  changes 
in  the  elasticity  of  the  muscle.  The  amplitude  of  the  contraction  depends 
upon  the  condition  of  the  muscle,  the  load,  the  strength  of  stimulus,  etc. 

Contraction  of  Non-striated  Muscle. — The  curve  obtained  by  regis- 
tration of  the  contraction  of  non-striated  muscle  shows  that  it  is  similar  in 
many  respects  to  that  of  the  striated  muscle,  except  that  the  duration  of  the 
former  is  considerably  longer  than  that  of  the  latter. 

Action  of  Successive  Stimuli. — If  a  series  of  successive  stimuli  be  applied 
to  a  muscle,  the  effect  will  be  different  according  to  the  rapidity  with  which 
they  follow  one  another.  If  the  second  stimulus  be  applied  at  the  termina- 
tion of  the  contraction  due  to  the  first  stimulus,  a  second  contraction  follows, 
similar  in  all  respects  to  the  first.  A  third  stimulus  produces  a  third  contrac- 
tion, and  so  on  until  the  muscle  becomes  exhausted.  If  the  second  stimulus 
be  applied  during  either  of  the  two  periods  of  the  first  contraction,  the  effects  of 
of  the  two  stimuli  will  be  added  together  and  the  second  contraction  will  add 
itself  to  the  first.  The  maximum  contraction  is  obtained  when  the  second 
stimulus  is  applied  -^  of  a  second  after  the  first. 

Tetanus. — When  a  series  of  stimuli  are  applied  to  a  muscle,  following 
one  another  with  median  rapidity,  the  muscle  does  not  get  time  to  relax  in 
the  intervals  of  stimulation,  but  remains  in  a  state  of  vibratory  contraction, 
which  may  be  regarded  as  incipient  tetanus,  or  clonus.  As  the  stimulation  in- 
creases in  frequency,  the  vibrations  become  invisible,  being  completely  fused 
together.  There  is,  nevertheless,  during  the  tetanic  condition  a  series  of  con- 
tinuous contractions  and  relaxations  taking  place.  After  a  varying  length 
of  time  the  muscle  becomes  fatigued,  and  not  withstanding  the  stimulation, 
begins  slowly  to  elongate.  The  number  of  stimuli  necessary  a  second  for 
the  production  of  tetanus  varies  in  different  animals — e.  g.,  2  to  3  for  muscles 
of  the  tortoise;  10  for  muscles  of  the  rabbi tts;  15  to  20  for  the  frog;  70  to  80 
for  birds;  330  to  340  for  insects. 


GENERAL   PHYSIOLOGY   OF   MUSCULAR   TISSUE.  53 

A  voluntary  contraction  in  man  may  be  regarded  as  a  state  of  tetanus, 
for  if  the  curve  of  a  voluntary  movement  be  examined,  it  will  be  found  to 
consist  of  intermittent  vibrations.  The  simplest  voluntary  movement  of  a 
muscle,  however  rapidly  it  may  take  place,  lasts  longer  than  a  single  muscular 
contraction  due  to  an  induction  shock.  The  most  rapid  voluntary  contrac- 
tion is  the  result  of  from  2.5  to  4  stimulations  a  second,  and  has  a  duration 
of  from  0.041  to  0.064  of  a  second.  A  continuous  voluntary  contraction  is 
an  incomplete  tetanus.  The  number  of  stimuli  sent  to  the  muscle  is  on  the 
average,  16  to  18  for  rapid  contractions,  8  to  12  for  slow  contractions. 

The  Production  of  Heat  and  Its  Relation  to  Mechanical  Work. — 

The  transformation  of  energy  which  takes  place  during  a  muscle  contraction, 
and  which  is  dependent  upon  chemic  changes  occurring  at  that  time,  mani- 
fests itself  as  heat  and  mechanical  work.  While  heat  is  being  evolved  con- 
tinuously during  the  passive  condition  of  muscles,  the  amount  of  heat  is 
largely  increased  during  general  muscle  contraction.  A  skeletal  muscle  of 
a  frog — e.  g.,  the  gastrocnemius — when  removed  from  the  body,  shows,  after 
tetanization,  an  increase  in  its  temperature  of  from  0.140  to  0.180  C,  and 
after  a  single  contraction  of  from  0.0010  to  0.0050  C.  While  every  muscular 
contraction  is  attended  by  an  incre.-  se  in  heat  production,  the  amount  so  pro- 
duced will  vary  in  accordance  with  certain  conditions — e.  g.,  tension,  work 
done,  fatigue,  circulation  of  blood   etc. 

Tension. — The  greater  the  tension  of  a  muscle,  the  greater,  other  conditions 
being  equal,  is  the  amouunt  of  heat  evolved.  When  the  ends  of  a  muscle 
are  fastened  so  that  no  shortening  is  possible  during  stimulation,  the  maximum 
of  heat  production  is  reached.  In  the  tetanic  state  the  great  increase  of 
temperature  is  due  to  the  tension  of  antagonistic  and  strongly  contracted 
muscles.  The  evolution  of  heat,  therefore,  bears  a  relation  to  the  resistance 
against  which  the  muscle  is  acting. 

Mechanical  Work. — If  a  muscle  contracts,  loaded  by  a  weight  just  sufficient 
to  elongate  it  to  its  original  length,  heat  is  evolved,  but  no  mechanical  work 
is  done,  all  the  energy  liberated  manifesting  itself  as  heat.  When  the  weight 
which  has  been  lifted  is  removed  from  the  muscle  at  the  height  of  contraction, 
external  work  is  done.  In  this  case  the  amount  of  heat  liberated  is  less, 
owing  to  the  work  done,  for  some  of  the  heat  generated  is  transformed  into 
mechanical  motion.  According  to  the  law  of  the  conservation  of  energy, 
the  amount  of  heat  disappearing  should  correspond  in  heat  units  to  the 
number  of  foot-pounds  produced  by  muscular  contraction. 

Muscle  Sound. — Providing  a  muscle  be  kept  in  a  state  of  tension  during 
its  contraction,  the  intermittent  variations  of  its  tension  cause  the  muscle 
to  emit  an  audible  sound.     If  the  muscle  be  tetanized  by  induction  shocks, 


54  HUMAN  PHYSIOLOGY. 

the  pitch  of  the  sound  corresponds  with  the  number  of  stimuli  a  second. 
A  voluntary  contraction  is  attended  by  a  tone  having  a  vibration  frequency 
of  about  thirty-six  a  second,  which  is,  however,  the  first  overtone  of  the  true 
muscle  tone,  which  is  caused  by  a  contraction  frequency  of  about  eighteen 
a  second.  This  low  tone  is  inaudible,  from  the  small  number  of  vibrations 
a  second. 

Muscle  Fatigue. — Prolonged  or  excessive  muscular  activity  is  followed  by 
a  diminution  in  the  power  of  producing  work  and  by  an  increase  in  the  dura- 
tion of  the  muscular  contractions.  Fatigue  is  accompanied  by  a  feeling  of 
stiffness,  soreness,  and  lassitude,  referable  to  the  muscles  themselves.  In  the 
early  stages  of  muscular  fatigue  the  contractions  increase  in  height  and  dura- 
tion, to  be  followed  by  a  progressive  decrease  in  height,  but  an  increase  in 
duration,  until  the  muscle  becomes  exhausted.  The  cause  of  the  fatigue  is 
the  production  and  accumulation  of  decomposition  products,  such  as  phos- 
phoric acid  and  phosphate  of  potassium,  C02,  etc.  A  fatigued  muscle  is 
rapidly  restored  by  the  injection  of  arterial  blood. 

Work  Done. — Muscles  are  machines  capable  of  doing  a  certain  amount  of 
work,  by  which  is  meant  the  raising  of  a  weight  against  gravity  or  the  over- 
coming of  some  resistance.  The  work  done  is  calculated  by  multiplying  the 
weight  by  the  distance  through  which  it  is  raised.  Thus,  if  a  muscle  shortens 
four  millimeters  and  raises  250  grams,  it  does  work  equal  to  1,000  milligram- 
meters,  or  one  gram-meter.  If  a  muscle  contracts  without  being  weighted,  no 
work  is  done.  Equally,  when  the  muscle  is  over-weighted  so  that  it  is  unable 
to  contract,  no  work  is  done.  The  amount  of  work  a  muscle  can  do  will 
depend  upon  the  area  of  its  transverse  section,  the  length  of  its  fibers,  and  the 
amount  of  the  weight.  The  amount  of  work  a  laborer  of  70  kilograms 
weight  performs  in  eight  hours  averages  105,605  kilogram-meters,  or  340.2 
foot-tons. 

SPECIAL  PHYSIOLOGY  OF  MUSCLES. 

The  individual  muscles  of  the  axial  and  appendicular  portions  of  the  body 
are  named  with  reference  to  their  shape,  action,  structure,  etc. — e.  g.,  deltoid, 
flexor,  penniform,  etc.  In  different  localities  a  group  of  muscles  having  a 
common  function  is  named  in  accordance  with  the  kind  of  motion  it  produces 
or  gives  rise  to — e.  g.,  groups  of  muscles  which  alternately  bend  or  straighten 
a  joint,  or  alternately  diminish  or  increase  the  angular  distance  between  two 
bones,  are  known  respectively  as  flexors  and  extensors;  such  muscle  groups  are 
in  association  with  ginglymus  joints.  Muscles  which  turn  the  bone  to  which 
they  are  attached  around  its  own  axis  without  producing  any  great  change  of 


SPECIAL   PHYSIOLOGY   OF    MUSCLES.  55 

position  are  known  as  rotators,  and  are  in  association  with  the  enarthrodial  or 
ball-and-socket  joints.  Muscles  which  impart  an  angular  movement  of  the 
extremities  to  and  from  the  median  line  of  the  body  are  termed  abductors  and 
adiuctors. 

In  addition  to  the  actions  of  individual  groups  of  muscles  in  causing  special 
movements  in  some  regions,  several  groups  of  muscles  are  coordinated  for  the 
accomplishment  of  certain  definite  functions — e.  g.,  muscles  of  respiration, 
mastication,  expression.  The  coordination  of  axial  and  appendicular  mus- 
cles enables  the  individual  to  assume  certain  postures,  such  as  standing  and 
sitting;  to  perform  various  acts  of  locomotion,  as  walking,  running,  swim- 
ming, etc. 

Levers. — The  function  or  special  mode  of  action  of  individual  muscles  can 
be  understood  only  when  the  bones  with  which  they  are  connected  are  re- 
garded as  levers  whose  fulcra  or  fixed  points  lie  in 

the  joints  where  the  movement  takes  place,  and  $ 

when    the    muscles    are  considered  as  sources    S  fl) 

of  power  for  imparting  movement  to  the  levers,     W  /\  * 

with    the    object   of    overcoming   resistance   or    p  a  I 

raising  weights.  »  -yy  p \  */ 

In  mechanics,  levers  of  three  kinds  or  orders  ♦ 

are  recognized,  according  to  the  relative  posi-  8        ■  (3) 

tion  of  the  fulcrum  or  axis  of  motion,  the  applied  r      / 

power,    and    the    weight    to    be    moved.     (See  ■f?IG-  5-     The  Three  Orders 
r  '  °  v  of  Levers. 

Fig-  5-)    , 

In  levers  of  the  first  order  the  fulcrum,  F,  lies  between  the  weight  or  resis- 
tance, W,  and  the  power  of  moving  force,  P.  The  distance  P-F  is  known  as 
the  power  arm,  the  distance  W-F  as  the  weight  arm.  As  an  example  of  this 
form  of  lever  in  the  human  body  may  be  mentioned: 

1.  The  elevation  of  the  trunk  from  the  flexed  position.  The  axis  of  move- 
ment, the  fulcrum,  lies  in  the  hip-joint;  the  weight,  that  of  the  trunk,  acting 
as  if  concentrated  at  its  center  of  gravity,  lies  between  the  shoulders;  the 
power,  the  contracting  muscles  attached  to  the  tuberosity  of  the  ischium. 
The  opposite  movement  is  equally  one  of  the  first  order,  but  the  relative 
positions  of  P  and  W  are  reversed. 

2.  The  skull  in  its  movements  backward  and  forward  upon  the  atlas.     In 
levers  of  the  second  order  the  weight  lies  between  the  power  and  the  fulcrum 
As  an  illustration  of  this  form  of  lever  may  be  mentioned: 

1.  The  depression  of  the  lower  jaw,  in  which  movement  the  fulcrum  is  the  tem- 
poromaxillary  articulation;  the  resistance,  the  tension  of  the  elevator 
muscles;  the  power,  the  contraction  of  the  depressor  muscles. 


56  HUMAN  PHYSIOLOGY. 

2.  The  raising  of  the  body  on  the  toes — F  being  the  toes,  W  the  weight  of 
the  body  acting  through  the  ankle,  P  the  gastrocnemius  muscle  acting  upon 
the  heel  bone. 

In  levers  of  the  third  order  the  power  is  applied  at  a  point  lying  between  the 
fulcum  and  the  weight.  As  examples  of  this  form  of  lever  may  be  mentioned: 
i.  The  flexion  of  the  forearm — F  being  the  elbow-joint,  P    the  contracting 

biceps  and  brachialis  anticus  muscles  applied  at  their  insertion,  W  the 

weight  of  the  forearm  and  hand. 
2.  The  extension  of  the  leg  on  the  thigh. 

When  levers  are  employed  in  mechanics,  the  object  aimed  at  is  the  over- 
coming of  a  great  resistance  by  the  application  of  a  small  force  acting  through 
a  great  space,  so  as  to  obtain  a  mechanical  advantage.  In  the  mechanism  of 
the  human  body  the  reverse  generally  obtains — viz.,  the  overcoming  of  a  small 
resistance  by  the  application  of  a  great  force  acting  through  a  small  space.  As 
a  result,  there  is  a  gain  in  the  extent  and  rapidity  of  movement  of  the  lever. 
The  power,  however,  owing  to  its  point  of  application,  acts  at  a  great  mechan- 
ical disadvantage  in  many  instances,  especially  in  levers  of  the  third  order. 

Postures. — Owing  to  its  system  of  joints,  levers,  and  muscles,  the  human 
body  can  assume  a  series  of  positions  of  equilibrium,  such  as  standing  and 
sitting,  to  which  the  name  posture  has  been  given.  In  order  that  the  body 
may  remain  in  a  state  of  stable  equilibrium  in  any  posture,  it  is  essential  that 
the  vertical  line  passing  through  the  center  of  gravity  shall  fall  within  the  base 
of  support. 

Standing  is  that  position  of  equilibrium  in  which  a  line  drawn  through  the 
center  of  gravity  falls  within  the  area  of  both  feet  placed  on  the  ground.  This 
position  is  maintained: 

i .  By  firmly  fixing  the  head  on  the  top  of  the  vertebral  column  by  the  action 
of  the  muscles  on  the  back  of  the  neck. 

2.  By  making  the  vertebral  column  rigid,  which  is  accomplished  by  the 
longissimus  dorsi  and  the  quadra tus  lumborum  muscles.  This  having 
been  accomplished,  the  center  of  gravity  falls  in  front  of  the  tenth  dorsal 
vertebra;  the  vertical  line  passing  through  this  point  falls  behind  the  line 
connecting  both  hip-joints.  In  consequence,  the  trunk  is  not  balanced  on 
the  hip-joints,  and  would  fall  backward  were  it  not  prevented  by  the  con- 
traction of  the  rectus  femoris  muscle  and  ligaments.  At  the  knees  and 
ankles  a  similar  balancing  of  the  parts  above  is  brought  about  by  the  action 
of  various  muscles.  When  the  entire  body  is  in  the  erect  or  military  posi- 
tion, the  arms  by  the  sides,  the  center  of  gravity  lies  between  the  sacrum 
and  the  last  lumbar  vertebra,  and  the  vertical  line  touches  the  ground  be- 
tween the  feet  and  within  the  base  of  support. 


SPECIAL   PHYSIOLOGY   OF   MUSCLES.  57 

Sitting  erect  is  a  condition  of  equilibrium  in  which  the  body  is  balanced 
on  the  tubera  ischii,  when  the  trunk  and  head  together  form  a  rigid  column. 
The  vertical  line  passes  between  the  tubera. 

Locomotion  is  the  act  of  transferring  the  body,  as  a  whole,  through  space, 
and  is  accomplished  by  the  combined  action  of  its  own  muscles.  The  acts 
involved  consist  of  walking,  running,  jumping,  etc. 

Walking  is  a  complicated  act,  involving  almost  all  the  voluntary  muscles  of 
the  body,  either  for  purposes  of  progression  or  for  balancing  the  head  and 
trunk,  and  may  be  defined  as  a  progression  in  a  forward  horizontal  direction, 
due  to  the  alternate  action  of  both  legs.  In  walking,  one  leg  becomes  for  the 
time  being,  the  active  or  supporting  leg,  carrying  the  trunk  and  head;  the 
other,  the  passive  but  progressive  leg,  to  become  in  turn  the  active  leg  when 
the  foot  touches  the  ground.  Each  leg,  therefore,  is  alternately  in  an  active 
and  a  passive  state. 

Running  is  distinguished  from  walking  by  the  fact  that,  at  a  given  moment, 
both  feet  are  off  the  ground  and  the  body  is  raised  in  the  air. 

While  the  limits  of  a  compend  do  not  permit  of  a  description  of  the  origin, 
insertion,  and  mode  of  action  of  the  individual  muscles  of  the  body,  it  has 
been  thought  desirable  to  call  attention  to  a  few  of  the  principle  muscles 
whose  function  it  is  to  produce  special  forms  of  movement,  as  well  as  loco- 
motion. (See  Fig.  6.)  The  erect  position  is  largely  maintained  by  the  fixa- 
tion of  the  spinal  column  and  the  balancing  of  the  head  upon  its  upper  ex- 
tremity; the  former  is  accompanied  by  the  erector  spince  muscle,  named  from 
its  function  and  its  fleshy  continuations,  situated  on  each  side  of  the  vertebral 
column.  Arising  from  the  pelvis  and  lumbar  vertebrae,  this  muscle  passes 
upward,  and  is  attached  by  its  continuations  to  all  the  vertebrae.  Its  action 
is  to  extend  the  vertebral  column  and  to  maintain  the  erect  position.  The 
head  is  balanced  upon  the  top  of  the  vertebral  column  by  the  combined  action 
of  the  trapezius  and  suboccipital  muscles  forming  the  nape  of  the  neck, 
and  by  the  sterno-cleido-mastoid  muscle.  This  latter  muscle  arises  from  the 
inner  third  of  the  clavicle  and  upper  border  of  the  sternum.  It  is  inserted  into 
the  temporal  bone  just  behind  the  ear.  Its  action  is  to  flex  the  head  laterally 
and  to  rotate  the  face  to  the  opposite  side.  When  both  muscles  act 
simultaneously,  the  head  and  neck  are  flexed  upon  the  thorax. 

The  temporal  and  masseter  muscles,  situated  at  the  side  of  the  head,  arise 
respectively  from  the  temporal  fossa  and  the  zygomatic  arch,  and  are  inserted 
into  the  ramus  of  the  lower  jaw.  Their  action  is  to  close  the  mouth  and  to 
assist  in  mastication.  The  occipito-frontalis,  the  orbicularis  palpebrarum,  and 
orbicularis  oris  muscles  are  largely  concerned  in  wrinkling  the  forehead,  closing 
the  eyes  and  mouth,  and  in  giving  various  expressions  to  the  face. 


58  HUMAN  PHYSIOLOGY. 

The  deltoid  is  a  thick,  triangular  muscle  covering  the  shoulder-joint. 
Arising  from  the  outer  third  of  the  clavicle,  the  acromial  process,  and  the 
spine  of  the  scapula,  its  fibers  converge  to  be  inserted  into  the  humerus 
just  above  its  middle  point.  Its  action  is  to  elevate  the  arm  through  a  right 
angle.  Owing  to  its  point  of  insertion  it  acts  as  a  lever  of  the  third  order, 
but,  notwithstanding  the  advantageous  points  of  insertion,  it  acts  at  a  con- 
siderable disadvantage,  owing  to  the  obliquity  of  its  direction. 

The  biceps  muscle,  situated  on  the  anterior  aspect  of  the  arm,  arises  from 
the  upper  border  of  the  glenoid  fossa  and  the  coracoid  process,  and  is  inserted 
into  the  radius  just  beyond  the  elbow-joint.  Its  action  is  to  flex  and  supinate 
the  forearm  and  to  place  it  in  the  most  favorable  position  for  striking  a  blow. 
When  the  forearm  is  fixed,  it  assists  in  flexing  the  arm,  as  in  climbing. 

The  triceps  muscle,  situated  on  the  back  of  the  arm,  arises  from  the  scapula 
and  the  posterior  surface  of  the  humerus,  and  is  inserted  in  the  olecranon 
process  of  the  ulna.  In  its  action  it  directly  antagonizes  the  biceps,  namely, 
extending  the  forearm.  In  so  doing  it  acts  as  a  lever  of  the  first  order.  The 
short  distance  between  the  muscular  insertion  and  the  fulcrum  causes  it  to 
act  at  a  great  mechanical  disadvantage,  but  there  is  a  corresponding  gain  in 
both  speed  and  range  of  movement.  The  muscles  of  the  forearm  are 
very  numerous.  Their  action  is  to  impart  to  the  forearm  and  hand  a 
variety  of  movements,  such  as  pronation,  supination,  flexion,  extension, 
rotation,  etc. 

The  pectoralis  major  and  pectoralis  minor  muscles  form  the  fleshy  masses 
of  the  breast.  Arising  from  the  inner  half  of  the  clavicle,  the  side  of  the 
sternum,  and  the  outer  surfaces  of  the  third,  fourth,  and  fifth  ribs  anteriorly, 
the  muscle-fibers  converge  to  be  inserted  into  the  humerus  and  coracoid 
process.  Their  combined  action  is  to  adduct,  flex  and  rotate  the  arm  inward 
and  to  draw  the  scapula  downward  and  forward,  movements  necessary  to  the 
folding  of  the  arms  across  the  chest. 

The  rectus  abdominis  and  the  obliquus  externus  assist  in  forming  the  abdom- 
inal walls. 

The  glutei  muscles  are  three  in  number,  are  arranged  in  layers,  and  form 
the  fleshy  masses  known  as  the  buttocks.  They  arise  from  the  side  of  the 
pelvis  and  are  attached  to  the  femur  in  the  neighborhood  of  the  great  trochan- 
ter. Their  action  is  to  extend  the  hips,  to  raise  the  body  from  the  stooping 
position,  and  to  assist  in  walking  by  firmly  holding  the  pelvis  on  the  thigh 
while  the  opposite  leg  is  advanced  in  the  forward  direction. 

The  rectus  femoris,  with  its  associates,  the  rectus  internus  and  rectus 
externus  and  the  crureus,  forms  the  fleshy  mass  on  the  anterior  surface  of 
the  thigh.  The  former  arises  from  the  anterior  part  of  the  ilium,  the  latter 
from  the  femur.     Their  common  tendon,  which  is  united  to  the  patella,  is 


SPECIAL   PHYSIOLOGY    OF    MUSCLES. 


59 


Fig.  6. — Superficial  Muscles  of  the  Body, 


60  HUMAN  PHYSIOLOGY. 

continued  as  the  ligamentum  patellae,  which  is  attached  to  the  upper  part  of 
the  tibia.  The  action  of  this  muscular  group  is  to  extend  the  leg,  to  flex 
the  thigh,  and  to  raise  the  entire  weight  of  the  body,  as  in  changing  from  the 
sitting  to  the  erect  position. 

The  biceps  femoris  muscle,  situated  on  the  outer  and  posterior  aspect  of  the 
thigh,  arises  from  the  tuber  ischii,  and  is  inserted  into  the  head  of  the  fibula. 

The  semimembranosus  and  the  semitendinosus  muscles,  situated  on  the 
inner  and  posterior  aspect  of  the  thigh,  are  inserted  into  the  head  of  the 
tibia.  Their  combined  action  is  to  extend  the  hips  and  to  flex  the  knee. 
Acting  from  below,  they  assist  in  raising  the  body  from  the  stooping  position. 

The  gastrocnemius  muscle  forms  the  enlargement  known  rs  the  calf  of  the 
leg.  It  arises  by  two  heads  from  the  condyles  of  the  fern  ax.  Its  tendon, 
the  tendo  Achillis,  is  inserted  into  the  posterior  surface  of  the  heel  bone. 
Its  action  is  to  extend  the  foot  and  to  raise  the  weight  of  the  body  in  walking 
and  running.  On  the  front  of  the  leg  are  numerous  muscles — e.  g.,  tibialis 
anticus,  peroneus  longus,  etc.,  the  action  of  which  is  to  flex  the  foot  and  to 
antagonize  the  gastrocnemius. 

PHYSIOLOGY  OF   NERVE  TISSUE. 

The  nerve  tissue,  which  unites  and  coordinates,  the  various  organs  and 
tissues  of  the  body  and  brings  the  individual  into  relationship  with  the  ex- 
ternal world,  is  arranged  anatomically  into  two  systems,  termed  the  encephalo 
or  cerebrospinal  and  the  sympathetic. 

The  encephalo  or  cerebro-spinal  system  consists  of: 
i.  The  brain  and  spinal  cord,  contained  within  the  cavities  of  the  cranium 
and  the  spinal  column  respectively,  and 

2.  The  cranial  and  spinal  nerves. 

The  sympathetic  system  consists  of: 

i.  A  double  chain  of  ganglia  situated  on  each  side  of  the  spinal  column  and 
extending  from  the  base  of  the  skull  to  the  tip  of  the  coccyx. 

2.  Various  collections  of  ganglia  situated  in  the  head,  face,  thorax,  abdomen, 
and  pelvis.  All  these  ganglia  are  united  by  an  elaborate  system  of  inter- 
communicating nerves,  many  of  which  are  connected  with  the  cerebro- 
spinal system. 

HISTOLOGY  OF  NERVE  TISSUE. 

The  Neuron. — The  nerve  tissue  has  been  resolved  by  the  investigations  of 
modern  histologists  into  a  single  morphologic  unit,  to  which  the  term  neuron 
has  been  applied.     The  entire  nervous  system  has  been  shown  to  be  but  an 


HISTOLOGY    OF    NERVE    TISSUE.  6l 

aggregate  of  an  infinite  number  of  neurons,  each  of  which  is  histologically 
distinct  and  independent.  Though  having  a  common  origin,  as  shown  by 
embryologic  investigations,  they  have  acquired  a  variety  of  forms  in  different 
parts  of  the  nervous  system  in  the  course  of  development.  The  old  concep- 
tion that  the  nervous  system  consists  of  two  distinct  histologic  elements, 
nerve-cells  and  nerve-fibers,  which  differed  not  only  in  their  mode  of  origin, 
but  also  in  their  properties,  their  relation  to  each  other,  and  their  functions, 
has  been  entirely  disproved. 

The  neuron,  or  neurologic  unit,  is  histologically  a  nerve-cell,  the  surface  of 
which  presents  a  greater  or  less  number  of  processes  in  varying  degrees  of 
differentiation.  As  represented  in  figure  7,  the  neuron  may  be  said  to  consist 
of:  (1)  The  nerve-cell,  neurocyte,  or  corpus;  (2)  the  axon,  or  nerve  process; 
(3)  the  end  tufts,  or  terminal  branches.  Though  these  three  main  histologic 
features  are  everywhere  recognizable,  they  exhibit  a  variety  of  secondary 
features  in  different  situations  in  accordance  with  peculiarities  of  function. 
Though  the  nerve-cell  and  the  nerve-fiber  are  but  part  of  the  same  neuron, 
it  is  convenient  at  present  to  describe  them  separately. 

The  Nerve-cell. — The  nerve-cell,  or  body  of  the  neuron,  presents  a 
variety  of  shapes  and  sizes  in  different  portions  of  the  nervous  system.  Origi- 
nally ovoid  in  shape,  it  has  acquired,  in  course  of  development,  peculiarities  of 
form  which  are  described  as  pyramidal,  stellate,  pear-shaped,  spindle-shaped, 
etc.  The  size  of  the  cell  varies  considerably,  the  smallest  having  a  diameter 
of  not  more  than  Woo  °f  an  inch,  the  largest  not  more  than  5^  of  an 
inch.  Each  cell  consists  of  granular,  striated  protoplasm,  containing  a  distinct 
vesicular  nucleus  and  a  well-defined  nucleolus.  A  cell  membrane  has  not  been 
observed.  From  the  surface  of  the  adult  cell  portions  of  the  protoplasm  are 
projected  in  various  directions,  which  portions,  rapidly  dividing  and  subdivid- 
ing form  a  series  of  branches,  termed  dendrites  or  dendrons.  In  some  situations 
the  ultimate  branches  of  the  dendrites  present  short  lateral  processes,  known 
as  lateral  buds,  or  gemmules,  which  impart  to  the  branches  a  feathery  appear- 
ance. This  characteristic  is  common  to  the  cells  of  the  cortex,  of  the  cere- 
brum, and  of  the  cerebellum.  The  ultimate  branches  of  the  dendrites, 
though  forming  an  intricate  feltwork,  never  anastomose  with  one  another, 
nor  unite  with  dendrites  of  adjoining  cells.  According  to  the  number  of 
axons,  nerve-cells  are  classified  as  monaxonic,  diaxonic,  polyaxonic.  Most 
of  the  cells  of  the  nervous  system  of  the  higher  vertebrates  are  monaxonic. 
In  the  ganglia  of  the  posterior  or  dorsal  roots  of  the  spinal  and  cranial  nerves, 
however,  they  are  diaxonic.  In  this  situation  the  axons,  emerging  from 
opposite  poles  of  the  cell,  either  remain  separate  and  pursue  opposite  direc- 
tions, or  unite  to  form  a  common  stem,  which  subsequently  divides  into  two 


62 


HUMAN  PHYSIOLOGY. 


branches,  which  then  pursue  opposite  directions.  (See  Fig.  7.)  The  nerve- 
cell  maintains  its  own  nutrition,  and  presides  over  that  of  the  dendrites  and 
the  axon  as  well.  If  the  latter  be  separated  in  any  part  of  its  course  from  the 
cell,  it  speedily  degenerates  and  dies. 


AXOM 


NEVE1LEHMA 


AXONE 


A  B 

Fig.  7. 
A.  Efferent  neuron.      B.  Afferent  neuron. 


The  axon,  or  nerve  process,  arises  from  a  cone-shaped  projection  from  the 
surface  of  the  cell,  and  is  the  first  outgrowth  from  its  protoplasm.  At  a  short 
distance  from  its  origin  it  becomes  markedly  differentiated  from  the  dendrites 
which  subsequently  develop.  It  is  characterized  by  a  sharp,  regular  outline, 
a  uniform  diameter,  and  a  hyaline  appearance.    In  structure,  the  axon  appears 


PHYSIOLOGY    OF    NERVE    TISSUE.  63 

to  consist  of  fine  fibrillar  embedded  in  a  clear,  protoplasmic  substance. 
Shafer  advocates  the  view  that  the  fibrillar  are  exceedingly  fine  tubes  filled 
with  fluid.  The  axon  varies  in  length  from  a  few  millimeters  to  100  cm.  In 
the  former  instance  the  axon,  at  a  short  distance  from  its  origin,  divides  into  a 
number  of  branches,  which  form  an  intricate  feltwork  in  the  neighborhood  of 
the  cell.  In  the  latter  instance  the  axon  continues  for  an  indefinite  distance 
as  an  individual  structure.  In  its  course,  however,  especially  in  the  central 
nervous  system,  it  gives  off  a  number  of  collateral  branches,  which  possess  all 
its  histologic  features.  The  long  axons  seem  to  bring  the  body  of  the  cell 
into  direct  relation  with  peripheral  organs,  or  with  more  or  less  remote  por- 
tions of  the  nervous  system,  thus  constituting  association  or  commissural 
fibers. 

The  more  or  less  elongated  axon  becomes  invested,  as  a  rule,  at  a  short 
distance  from  the  cell  with  nucleated  oblong  cells,  which  subsequently  be- 
come modified  and  constitute  a  medullary  or  myelin  sheath.  This  is  in- 
vested by  a  thin,  cellular  membrane — the  neurilemma.  These  three  struc- 
tures thus  constitute  what  is  known  as  a  medullated  nerve-fiber.  In  the 
central  nervous  system  the  outer  sheath  is  frequently  absent.  In  the  sympa- 
thetic system  the  myelin  is  frequently  absent,  though  the  axon  is  inclosed  by 
the  neurilemma,  thus  constituting  a  non-medulla  ted  nerve-fiber. 

The  end  tufts  or  terminal  organs  are  formed  by  the  splitting  of  the  axon  into 
a  number  of  filaments,  which  remain  independent  of  one  another  and  are  free 
from  the  medullary  investment.  The  histologic  peculiarities  of  the  terminal 
organs  vary  in  different  situations,  and  in  many  instances  are  quite  complex 
and  characteristic.  In  peripheral  organs,  as  muscles,  glands,  blood-vessels, 
skin,  mucous  membrane,  the  tufts  are  in  direct  organic  connection  with  their 
cellular  elements.  In  the  central  nervous  system  the  tufts  are  in  more  or  less 
intimate  relation  with  the  dendrites  of  adjacent  neurons. 

Nerve-fibers. — The  axons  with  their  secondary  investments  together  con- 
stitute the  nerve-fibers,  and  according  as  they  possess  or  do  not  possess  the 
medullary  sheath,  they  may  be  divided  into  two  groups — viz.,  medullated 
and  non-medullated  fibers. 

Medullated  Nerve-fibers. — These  consist  for  the  most  part  of  three  dis- 
tinct structures.: 

1.  An  external  investing  sheath,  tubular  in  shape,  termed  the  neurilemma. 

2.  An  intermediate  semifluid  substance — the  medulla  or  myelin. 

3.  An  internal  dark  thread — the  axis-cylinder. 

The  neurilemma  is  a  thin,  transparent,  homogeneous  membrane  closely 
adherent  to  the  medulla.  Owing  to  its  colorless  appearance,  it  can  be  seen 
only  with  difficulty  in  the  recent  condition.     When  treated  with  various 


64  HUMAN  PHYSIOLOGY. 

reagents,  it  becomes  distinct.  Physically,  it  is  quite  resistant  and  elastic. 
Its  function  is  doubtless  that  of  a  protective  agent  to  the  structures  within. 

The  medulla,  myelin,  or  white  substance  of  Schwann  completely  fills  the 
neurilemma  and  closely  invests  the  axis-cylinder.  In  the  recent  condition 
the  medulla  is  clear,  homogeneous,  semifluid,  and  highly  refracting.  In 
composition  it  is  oleaginous.  When  the  nerve  is  treated  with  various  re- 
agents which  alter  its  composition,  the  medulla  becomes  opaque  and  imparts 
to  the  nerve  a  white,  glistening  appearance.  The  function  of  the  medulla  is 
quite  unknown. 

At  intervals  of  about  seventy-five  times  its  diameter  the  medullated  nerve- 
fiber  undergoes  a  remarkable  diminution  in  size,  due  to  an  interruption  of  the 
medullary  substance,  so  that  the  neurilemma  lies  directly  on  the  axis-cylinder. 
These  constrictions,  or  nodes  of  Ranvier,  taking  their  name  from  their  dis- 
coverer, occur  at  regular  intervals  along  the  course  of  the  nerve,  separating  it 
into  a  series  of  segments.  The  portion  between  the  nodes  is  termed  the  inter- 
nodal  segment.  It  has  been  suggested  that  in  consequence  of  the  absence  of 
the  myelin  at  these  nodes,  a  free  exchange  of  nutritive  material  and  decompo- 
sition products  can  take  place  between  the  axis-cylinder  and  the  surrounding 
plasma. 

The  axis-cylinder,  or  axon,  the  direct  outgrowth  of  the  nerve-cell,  is  the 
most  essential  element  of  the  nerve-fiber,  as  it  alone  is  uniformly  continuous 
throughout.  In  the  natural  condition  it  is  transparent  and  invisible;  but 
when  treated  with  proper  reagents,  it  presents  itself  as  a  pale,  granular, 
flattened  band,  more  or  less  solid  and  somewhat  elastic.  It  is  albuminous  in 
composition.  With  high  magnification  the  axis  presents  a  longitudinal 
striation,  indicating  a  fibrillar  structure.  The  fibrillse  appear  to  be  united  by 
an  intervening  cement  substance. 

Non-medullated  Nerve-fibers. — These  consist,  for  the  most  part,  only  of 
the  axis-cylinder,  though  in  some  portions  of  the  nervous  system  a  neurilemma 
is  also  present.  Though  much  less  abundant  than  the  former  variety,  they 
are  distributed  largely  throughout  the  nervous  system,  but  are  particularly 
abundant  in  the  sympathetic  system.  Owing  to  the  absence  of  a  medulla, 
they  present  a  rather  pale  or  grayish  appearance. 

Structure  of  Nerve  Trunks.  —  After  their  emergence  from  the  brain  and 
spinal  cord,  the  nerve-fibers  become  bound  together,  by  connective  tissue,  into 
the  form  of  continuous  bundles,  which  connect  the  brain  and  cord  with  all  the 
remaining  structures  of  the  body.  The  bundles  are  technically  known  as 
nerve  trunks  or  nerves.  Each  nerve  is  invested  by  a  thick  layer  of  lamellated 
connective  tissue,  known  as  the  epineurium.  A  transverse  section  of  a  nerve 
shows  (see  Fig.  8)  that  it  is  made  up  of  a  number  of  small  bundles  of  fibers, 


HIASIOLOGY    OF    NERVE    TISSUE. 


each  of  which  possesses  a  separate  investment  of  connective  tissue — the  peri- 
neurium. Within  this  membrane  the  nerve-fibers  are  supported  by  a  fine 
stroma — the  endoneurium.  After  pursuing  a  longer  or  shorter  course,  the 
nerve  trunk  gives  off  branches,  which  interlace  very  freely  with  neighboring 
branches,  forming  plexuses,  the  fibers  of  which  are  distributed  to  associated 
organs  and  regions  of  the  body.  From  their  origin  to  their  termination,  how- 
ever, nerve-fibers  retain  their  individuality,  and  never  become  blended  with 
adjoining  fibers. 


.  '     '    '■  .  •'     .  "■ 

1 


&£L 


Fig.  8. — Transverse  Section  of  a  Nerve  (Median). 
ep.  Epineurium.     pe.  Perineurium,     ed.  Endoneurium. 

As  nerves  pass  from  their  origin  to  their  peripheral  terminations,  they  give 
off  a  number  of  branches,  each  of  which  becomes  invested  with  a  lamellated 
sheath — an  offshoot  from  that  investing  the  parent  trunk.  This  division  of 
nerve  bundles  and  sheath  continues  throughout  all  the  branches  down  to  the 
ultimate  nerve-fibers,  each  of  which  is  surrounded  by  a  sheath  of  its  own, 
consisting  of  a  single  layer  of  endothelial  cells.  This  delicate  transparent 
membrane,  the  sheath  of  Henle,  is  separated  from  the  nerve-fiber  by  a  con- 
siderable space,  in  which  is  contained  lymph  destined  for  the  nutrition  of  the 
fiber.  Near  their  ultimate  terminations  the  nerve-fibers  themselves  undergo 
division,  so  that  a  single  fiber  may  give  origin  to  a  number  of  branches, 
each  of  which  contains  a  portion  of  the  parent  axis-cylinder  and  myelin. 

5 


66  HUMAN  PHYSIOLOGY. 

CLASSIFICATION  OF  NERVES. 

Nerves  are  channels  of  communication  between  the  brain  and  spinal  cord, 
on  the  one  hand,  and  the  muscles,  glands,  blood-vessels,  skin,  mucus  mem- 
brane, viscera,  etc.,  on  the  other.  Some  of  the  nerve-fibers  serve  for  the 
t  ransmission  of  nerve  energy  or  nerve  impulses  from  the  brain  and  spinal  cord 
to  certain  peripheral  organs,  and  so  increase  or  retard  their  activities;  others 
serve  for  the  transmission  of  nerve  energy  from  certain  peripheral  organs  to 
the  brain  and  spinal  cord,  which  gives  rise  to  sensations  or  other  modes  of 
nerve  activity.  The  former  are  termed  efferent  or  centrifugal  nerves;  the 
latter  are  termed  afferent  or  centripetal  nerves. 

The  efferent  nerves  may  be  classified,  in  accordance  with  the  character- 
istic form  of  activity  to  which  they  give  rise,  into  several  groups,  as  follows : 
i.  Muscle  or  motor  nerves,  those  which  convey  nerve  energy  or  nerve  impulses 

to  muscles  and  give  rise  to  muscle  contraction. 

2.  Gland  or  secretory  nerves,  those  which  convey  nerve  impulses  to  glands, 
and  cause  the  formation  of  the  secretion  peculiar  to  the  gland. 

3.  Vascular  or  vaso-motor  nerves,  those  which  convey  nerve  impulses  to  blood- 
vessels, and  cause,  either  by  stimulation  or  inhibition  of  the  mechanism  of 
their  walls,  a  contraction  (vaso-constrictors)  or  dilatation  (vaso-dilatators) 
of  the  vessel. 

4.  Inhibitor  nerves,  those  conveying  nerve  impulses  that  cause  a  slowing  or 
complete  cessation  of  the  rhythmic  action  of  organs. 

5.  Accelerator  nerves,  those  conveying  impulses  that  cause  an  increase  in  the 
rhythmic  action  of  certain  organs. 

The  afferent  nerves  may  also  be  classified,  in  accordance  with  the  charac- 
ter of  the  sensations  or  other  modes  of  nerve  activity  to  which  they  give  rise, 
into  several  groups,  as  follows: 

1.  Sensorifacient  nerves,  those  conveying  nerve  impulses  that  give  rise  in 
the  brain  to  conscious  sensations.     They  may  be  subdivided  into — 

(a)  Nerves  of  special  sense — e.  g.,  olfactory,  optic,  auditory,  gustatory, 
tactile,  thermal,  sensory,  muscle — those  which  give  rise  to  olfactory,  optic, 
auditory,  gustatory,  tactile,  thermic,  painful,  and  muscle  sensations. 

(b)  Nerves  of  general  sense — e.  g.,  the  visceral  afferent  nerves — those 
which  give  rise  normally  to  vague  and  scarcely  perceptible  sensations,  such 
as  the  general  sensations  of  well-being  or  discomfort,  thirst,  fatigue,  sex, 
want  of  air,  etc. 

2.  Reflex  nerves,  those  which  convey  nerve  impulses  to  the  nerve  centers  and 
cause  a  discharge  and  transmission  of  nerve  impulses  outward  through 
efferent  nerves  to  muscles,  glands,  or  blood-vessels,  and  thus  influence  their 


PHYSIOLOGY   OF   NERVE   TISSUE.  67 

activity.  It  is  quite  probable  that  one  and  the  same  nerve  may  subserve 
both  sensational  and  reflex  action,  owing  to  the  collateral  branches  which  are 
given  off  from  the  posterior  roots  as  they  ascend  the  posterior  column  of  the 
cord. 
3.  Inhibitor  nerves,  those  which  are  capable  reflexly  of  retarding  or  inhibiting 
the  activity  of  either  nerve  centers  or  peripheral  organs. 

The  Terminal  Endings  of  Nerves. — The  efferent  nerves,  as  they  approach 
their  ultimate  terminations,  lose  both  the  neurilemma  and  medullary  sheath. 
The  axis-cylinder  then  divides  into  a  number  of  tufts  or  branches,  which 
become  directly  and  intimately  connected  with  the  tissue  cells.  The  particular 
mode  of  termination  varies  in  different  situations.  These  terminations  are 
generally  spoken  of  as  "end  organs." 

In  the  skeletal  muscles  the  nerve-fiber  loses  both  neurilemma  and  myelin 
sheath  at  the  point  where  it  comes  in  contact  with  the  muscle-fiber.  After 
penetrating  the  sarcolemma,  the  axis-cylinder  breaks  up  into  small  branches 
with  bulbous  extremities,  forming  the  so-called  "motor-plate,"  which  rests 
directly  on  a  disc  of  granular  material  containing  oval,  vesicular  nuclei. 
Each  muscle-fiber  possesses  an  individual  end-plate. 

In  the  visceral  muscles  the  terminal  nerve-fibers  form  a  plexus  around  the 
muscle-fibers,  and  become  organically  connected  with  them.  In  the  glands 
the  nerve  fibers  have  been  traced  directly  to  their  secreting  cells.  The  exact 
mode  of  their  termination  and  connection  with  the  cells  has  not  been  clearly 
determined. 

The  afferent  nerves,  as  they  approach  their  peripheral  terminations,  become 
connected  in  like  manner  with  end  organs,  which,  in  some  distances,  are 
extremely  complex,  such  as  those  found  in  the  eye  (retina),  the  internal  ear, 
the  nose,  and  the  tongue.  (A  consideration  of  these  end  organs  will  be  found 
in  the  chapters  devoted  to  the  organs  of  which  they  form  a  part.)  The  end 
organs  of  the  skin  and  mucous  membranes  present  a  variety  of  forms,  and 
may  be  classified  as  follows: 

1.  Free  endings  in  the  epithelium  of  the  skin,   mucous  membrane,  and 
cornea. 

2.  Tactile  cells  of  Merkel  in  the  epidermis. 

3.  Tactile  corpuscles  in  the  papillae  of  the  true  skin. 

4.  Pacinian  corpuscles  found  attached  to  the  nerves  of  the  hands  and  feet, 
to  the  intercostal  nerves,  and  to  nerves  in  other  situations. 

5.  End  bulbs  of  Krause  in  the  conjunctiva,  penis,  clitoris,  etc. 

The  end  organs  of  the  afferent  nerves  are  specialized,  highly  irritable 
structures  placed  between  the  nerve-fibers  and  the  surface  of  the  body. 
They  are  especially  adapted  for  the  reception  of  those  external  forces  tech- 


68  HUMAN  PHYSIOLOGY. 

nically  known  as  stimuli,  and  for  the  liberation  of  energy  capable  of  exciting 
the  nerve-fiber  to  activity. 

Relation  of  Spinal  Nerves  to  the  Spinal  Cord. — The  nerves  in  con- 
nection with  the  spinal  cord  are  thirty-one  in  number,  on  each  side  and  have 
two  roots  of  origin,  an  anterior  and  a  posterior,  which  arise  from  the  anterior 
and  posterior  surfaces  of  the  cord  respectively.  They  are  more  properly 
termed  ventral  and  dorsal  roots.  The  dorsal  roots  present,  near  their  entrance 
into  the  cord,  an  enlargement  termed  a  ganglion.  Beyond  the  spinal  canal 
these  two  roots  unite  to  form  the  ordinary  spinal  nerve.  Some  of  the  nerves 
in  connection  with  the  base  of  the  brain  also  present  a  ganglionic  enlargement, 
and  may,  therefore,  be  regarded  physiologically  as  dorsal  nerves,  while 
others  may  be  regarded  as  ventral  nerves. 

Experimentally,  it  has  been  determined  that  the  anterior  or  ventral  roots 
contain  all  the  efferent  fibers,  the  posterior  or  dorsal  roots  all  the  afferent 
fibers.     The  proofs  in  support  of  this  view  are  as  follows: 

Stimulation  of  the  ventral  roots  produces: 
i.  Convulsive  movements  of  muscles. 

2.  The  formation  of  a  secretion  of  glands. 

3.  Changes  in  the  caliber  of  blood-vessels. 

4.  Inhibition  of  the  rhythmic  activity  of  certain  organs. 
Divisions  of  these  roots  is  followed  by: 

1.  Loss  of  muscular  movement  (paralysis  of  motion). 

2.  Cessation  of  secretion. 

3.  Cessation  of  vascular  changes. 
Stimulation  of  the  dorsal  roots  causes: 

1.  Reflex  activities. 

2.  Conscious  sensations. 

3.  Inhibition  of  the  rhythmic  activity  of  certain  organs. 
Division  of  these  roots  is  followed  by: 

1.  Loss  of  reflex  activities,  and 

2.  Loss  of  sensation  in  all  parts  to  which  they  are  distributed. 

The  ventral  roots  are,  therefore,  efferent  in  function,  transmitting  nerve 
impulses  from  the  nerve  centers  to  the  periphery.  The  dorsal  roots  are 
afferent  in  function,  transmitting  nerve  impulses  from  the  general  periphery 
to  the  nerve  centers. 

Development  and  Nutrition  of  Nerves. — The  efferent  nerve-fibers, 

which  constitute  some  of  the  cranial  nerves  and  all  the  ventral  roots  of  the 
spinal  nerves,  have  their  origin  in  cells  located  in  the  gray  matter  beneath  the 
aqueduct  of  Sylvius,  beneath  the  floor  of  the  fourth  ventricle  and  in  the 
anterior  horns  of  the  gray  matter  of  the  spinal  cord.     These  cells  are  the  modi- 


PHYSIOLOGY   OF   NERVE   TISSUE. 


69 


fied  descendants  of  independent,  oval,  pear-shaped  cells — the  neuroblasts — 
which  migrate  from  the  medullary  tube.  As  they  approach  the  surface  of  the 
cord  their  axons  are  directed  toward  the  ventral  surface,  which  eventually 
they  pierce.  Emerging  from  the  cord,  the  axons  continue  to  grow,  and  be- 
come invested  with  the  myelin  sheath  and  neurilemma,  thus  constituting 
the  ventral  roots. 

The  afferent  nerve-fibers,  which  constitute  some  of  the  cranial  nerves  and 
s  11  the  dorsal  roots  of  the  spinal  nerves,  develop  outside  of  the  central  nervous 
system  and  only  subsequently  become  connected  with  it.     (See  Fig.   9.) 


terior 
Jloct 


Fig.  9. 


£oot 


-Diagram   Showing  the  Mode  of  Origin  of  the  Ventral  and  Dorsal 

Roots. 


At  the  time  of  the  closure  of  the  medullary  tube  a  band  or  ridge  of  epithelial 
tissue  develops  near  the  dorsal  surface,  which,  becoming  segmented,  moves 
outward  and  forms  the  rudimentary  spinal  ganglia.  The  cells  in  this 
situation  develop  two  axons,  one  from  each  end  of  the  cell,  which  pass  in 
opposite  directions,  one  toward  the  spinal  cord,  the  other  toward  the  per- 
iphery. In  the  adult  condition  the  two  axons  shift  their  position,  unite,  and 
form  a  T-shaped  process,  after  which  a  division  into  two  branches  again 
takes  place.  In  the  ganglia  of  all  the  sensoricranial  and  sensorispinal 
nerves  the  cells  have  this  histologic  peculiarity. 


70  HUMAN  PHYSIOLOGY. 

Nerve  Degeneration. — If  any  one  of  the  cranial  or  spinal  nerves  be  divided 
in  any  portion  of  its  course,  the  part  in  connection  with  the  periphery  in  a 
short  time  exhibits  certain  structural  changes,  to  which  the  term  degeneration 
is  applied.  The  portion  in  connection  with  the  brain  or  cord  retains  its  nor- 
mal condition.  The  degenerative  process  begins  simultaneously  throughout 
the  entire  course  of  the  nerve,  and  consists  in  a  disintegration  and  reduction 
of  the  medulla  and  axis-cylinder  into  nuclei,  drops  of  myelin,  and  fat,  which 
in  time  disappear  through  absorption,  leaving  the  neurilemma  intact.  Coin- 
cident with  these  structural  changes  there  is  a  progressive  alteration  and 
diminution  in  the  excitability  of  the  nerve.  Inasmuch  as  the  central  portion 
of  the  nerve,  which  retains  its  connection  with  the  nerve-cell,  remains  histo- 
logically normal,  it  has  been  assumed  that  the  nerve-cells  exert  over  the  entire 
course  of  the  nerve-fibers  a  nutritive  or  a  trophic  influence.  This  idea  has 
been  greatly  strengthened  since  the  discovery  that  the  axis-cylinder,  or  the 
axon,  has  its  origin  in  and  is  a  direct  outgrowth  of  the  cell.  When  separated 
from  the  parent  cell,  the  fiber  appears  to  be  incapable  of  itself  of  maintaining 
its  nutrition. 

The  relation  of  the  nerve-cells  to  the  nerve-fibers,  in  reference  to  their 
nutrition,  is  demonstrated  by  the  results  which  follow  section  of  the  ventral 
and  dorsal  roots  of  the  spinal  nerves.  If  the  anterior  root  alone  be  divided, 
the  degenerative  process  is  confined  to  the  peripheral  portion,  the  central 
portion  remaining  normal.  If  the  posterior  root  be  divided  on  the  peripheral 
side  of  the  ganglion,  degeneration  takes  place  only  in  the  peripheral  portion  of 
the  nerve.  If  the  root  be  divided  between  the  ganglion  and  the  cord,  degener- 
ation takes  place  only  in  the  central  portion  of  the  root.  From  these  facts  it 
is  evident  that  the  trophic  centers  for  the  ventral  and  dorsal  roots  He  in  the 
spinal  cord  and  spinal  nerve  ganglia,  respectively,  or,  in  other  words,  in  the 
cells  of  which  they  are  an  integral  part.  The  structural  changes  which  nerves 
undergo  after  separation  from  their  centers  are  degenerative  in  character, 
and  the  process  is  usually  spoken  of,  after  its  discoverer,  as  the  Wallerian 
degeneration. 

When  the  degeneration  of  the  efferent  nerves  is  completed,  the  structures 
to  which  they  are  distributed,  especially  the  muscles,  undergo  an  atrophic  or 
fatty  degeneration,  with  a  change  or  loss  of  their  irritability.  This  is,  ap- 
parently, not  to  be  attributed  merely  to  inactivity,  but  rather  to  a  loss  of 
nerve  influences,  inasmuch  as  inactivity  merely  leads  to  atrophy  and  not  to 
degeneration. 

Reactions  of  Degeneration. — In  consequence  of  the  degeneration  and 
changes  in  irritability  which  occur  in  nerves  when  separated  from  their 
centers  and  in  muscles  when  separated  from  their  related  nerves,  either 


PHYSIOLOGY   OF   NERVE   TISSUE.  7 1 

experimentally  or  as  the  result  of  disease,  the  response  of  these  structures  to 
the  induced  and  the  make-and-break  of  the  constant  currents  differs  from 
that  observed  in  the  physiologic  condition.  The  facts  observed  under  the 
application  of  these  two  forms  of  electricity  are  of  the  greatest  importance  in 
the  diagnosis  and  therapeutics  of  the  precedent  lesions.  The  principal 
difference  of  behavior  is  observed  in  the  muscles,  which  exhibit  a  diminished 
or  abolished  excitability  to  the  induced  current,  while  at  the  same  time  mani- 
festing an  increased  excitability  to  the  constant  current;  so  much  so  is  this  the 
case  that  a  closing  contraction  is  just  as  likely  to  occur  at  the  positive  as  at 
the  negative  pole.  This  peculiarity  of  the  muscle  response  is  termed  the 
reaction  of  degeneration.  The  synchronous  diminished  excitability  of  the 
nerves  is  the  same  for  either  current.  The  term ' '  partial  reaction  of  degenera- 
tion" is  used  when  there  is  a  normal  reaction  of  the  nerves,  with  the  degen- 
erative reaction  of  the  muscles.  This  condition  is  observed  in  progressive 
muscular  atrophy. 

Reflex  Action. — Inasmuch  as  many  of  the  muscle  movements  of  the  body, 
as  well  as  the  formation  and  discharge  of  secretions  from  glands,  variations 
in  the  caliber  of  blood-vessels,  inhibition  and  acceleration  in  the  activity  of 
various  organs,  are  the  result  of  stimulations  of  the  terminal  organs  of  afferent 
nerves,  they  are  termed,  for  convenience,  reflex  actions,  and,  as  they  take 
place  independently  of  the  brain  or  of  volitional  impulses,  they  are  also  termed 
involuntary  actions.  As  many  of  the  processes  to  be  described  in  succeeding 
chapters  are  of  this  character,  requiring  for  their  performance  the  cooperation 
of  several  organs  and  tissues  associated  through  the  intermediation  of  the 
nervous  system,  it  seems  advisable  to  consider  briefly,  in  this  connection,  the 
parts  involved  in  a  reflex  action,  as  well  as  their  mode  of  action.  As  shown 
in  figure  10,  the  necessary  structures  are  as  follows: 
i.  A  receptive  surface,  skin,  mucous  membrane,  sense  organ,  etc. 

2.  An  afferent  nerve. 

3.  An  emissive  cell,  from  which  arises 

4.  An  afferent  nerve,  distributed  to  a  responsive  organ,  as, 

5.  Muscle,  gland,  blood-vessel,  etc. 

Such  a  combination  of  structures  constitutes  a  reflex  mechanism  or  arc  the 
nerve  portion  of  which  is  composed  of  but  two  neurons — an  afferent  and  an 
efferent.  An  arc  of  this  simplicity  would  of  necessity  subserve  but  a  simple 
movement.  The  majority  of  reflex  activities,  however,  are  extremely  com- 
plex, and  involve  the  cooperation  and  coordination  of  a  number  of  structures 
frequently  situated  at  distances  more  or  less  remote  from  one  another.  This 
implies  that  a  number  of  neurons  are  associated  in  function.  The  afferent 
neurons  are  brought  into  relation  with  the  dendrites  of  the  efferent  neurons  by 


72 


HUMAN  PHYSIOLOGY. 


the  end  tufts  of  the  collateral  branches,  which  may  extend  for  some  distance 
up  and  down  the  cord  before  passing  into  the  various  segments. 

For  the  excitation  of  a  reflex  action  it  is  essential  that  the  stimulus  applied 
to  the  sentient  surface  be  of  an  intensity  sufficient  to  develop  in  the  terminals 
of  the  afferent  nerve  a  series  of  nerve  impulses,  which,  raveling  inward,  will 
be  distributed  to  and  received  by  the  dendrites  of  the  emissive  or  motor  cell. 
With  the  reception  of  these  impulses  there  is  apparently  a  disturbance  of  the 
equilibrium  of  its  molecules,  a  liberation  of  energy,  and,  in  consequence,  a 


Fig.  io. — Diagram  Illustrating  Reflex  Action. — (Kirke.) 
S.  Receptive  surface  from  which  proceeds  the  afferent  nerve.     M.  C.  Motor  or 
emissive  cell  giving  origin  to  efferent  nerve  which  terminates  in  M.     M.  Motor 
organ.     G.  Ganglion  cell  on  afferent  nerve. 


transmission  outward  of  impulses  through  the  efferent  nerve  to  muscle,  gland, 
or  blood-vessel,  separately  or  collectively,  with  the  production  of  muscular 
contraction,  glandular  secretion,  vascular  dilatation  or  contraction,  etc.  The 
reflex  actions  take  place,  for  the  most  part,  through  the  spinal  cord  and  med- 
ulla oblongata,  which,  in  virtue  of  their  contained  centers,  coordinate  the 
various  organs  and  tissues  concerned  in  the  performance  of  the  organic  func- 
tions. The  movements  of  mastication;  the  secretion  of  saliva;  the  muscular, 
glandular,  and  vascular  phenomena  of  gastric  and  intestinal  digestion;  the 
vascular  and  respiratory  movements;  the  mechanism  of  micturition,  etc.,  are 
illustrations  of  reflex  activity. 


PHYSIOLOGY   OF   NERVE   TISSUE.  73 

PHYSIOLOGIC  PROPERTIES  OF  NERVES. 

Nerve  Irritability  or  Excitability  and  Conductivity. — These  terms  are 
employed  lo  express  that  condition  of  a  nerve  which  enables  it  to  develop  and 
to  conduct  nerve  impulses  from  the  center  to  the  periphery,  from  the  periphery 
to  the  center,  in  response  to  the  action  of  stimuli.  A  nerve  is  said  to  be  ex- 
citable or  irritable  as  long  it  possesses  these  capabilities  or  properties.  For  the 
manifestation  of  these  properties  the  nerve  must  retain  a  state  of  physical  and 
chemic  integrity;  it  must  undergo  no  change  in  structure  or  chemic  compo- 
sition. The  irritability  of  an  efferent  nerve  is  demonstrated  by  the  contrac- 
tion of  a  muscle,  by  the  secretion  of  a  gland,  or  by  a  change  in  the  caliber  of 
a  blood-vessel,  whenever  a  corresponding  nerve  is  stimulated.  The  irrita- 
bility of  an  afferent  nerve  is  demonstrated  by  the  production  of  a  sensation  or  a 
reflex  action  whenever  it  is  stimulated.  The  irritability  of  nerves  continues 
for  a  certain  period  of  time  after  separation  from  the  nerve  centers  and  even 
after  the  death  of  the  animal,  varying  in  different  classes  of  animals.  In  the 
warm-blooded  animals,  in  which  the  nutritive  changes  take  place  with  great 
rapidity,  the  irritability  soon  disappears — a  result  due  to  disintegrative 
changes  in  the  nerve,  caused  by  the  withdrawal  of  the  blood-supply.  In  cold- 
blooded animals,  on  the  contrary,  in  which  the  nutritive  changes  take  place 
relatively  slowly,  the  irritability  lasts,  under  favorable  conditions,  for  a  con- 
siderable time.  Other  tissues  besides  nerves  possess  irritability,  that  is,  the 
property  of  responding  to  the  action  of  stimuli — e.  g.,  glands  and  muscles, 
which  respond  by  the  production  of  a  secretion  or  a  contraction. 

Independence  of  Tissue  Irritability. — The  irritability  of  nerves  is 
distinct  and  independent  of  the  irritability  of  muscles  and  glands,  as  shown 
by  the  fact  that  it  persists  in  each  a  variable  length  of  time  after  their  histo- 
logic connections  have  been  impaired  or  destroyed  by  the  introduction  of 
various  chemic  agents  into  the  circulation.  Curara,  for  example,  induces  a 
state  of  complete  paralysis  by  modifying  or  depressing  the  conductivity  of  the 
end  organs  of  the  nerves  just  where  they  come  in  contact  with  the  muscles 
without  impairing  the  irritability  of  either  nerve  trunks  or  muscles.  Atropin 
induces  complete  suspension  of  glandular  activity  by  impairing  the  terminal 
organs  of  the  secretor  nerves  just  where  they  come  into  relation  with  the  gland 
cells,  without  destroying  the  irritability  of  either  gland  or  nerve. 

Stimuli  of  Nerves. — Nerves  do  not  possess  the  power  of  spontaneously 
generating  and  propagating  nerve  impulses;  they  can  be  aroused  to  activity 
only  by  the  action  of  an  extraneural  stimulus.  In  the  living  condition  the 
stimuli  capable  of  throwing  the  nerve  into  an  active  condition  act  for  the  most 
part  on  either  the  central  or  peripheral  end  of  the  nerve.  In  the  case  of  mo- 
tor nerves  the  stimulus  to  the  excitation,  originating  in  some  molecular  dis- 


74  HUMAN  PHYSIOLOGY. 

turbance  in  the  nerve-cells,  acts  upon  the  nerve-fibers  in  connection  with 
them.  In  the  case  of  sensor  or  afferent  nerves  the  stimuli  act  upon  the  pecu- 
liar end  organs  with  which  the  sensor  nerves  are  in  connection,  which  in  turn 
excite  the  nerve-fibers.  Experimentally,  it  can  be  demonstrated  that  nerves 
can  be  excited  by  a  sufficiently  powerful  stimulus  applied  in  any  part  of  their 
extent. 

Nerves  respond  to  stimulation  according  to  their  habitual  function;  thus, 
stimulation  of  a  sensor  nerve,  if  sufficiently  strong,  results  in  the  sensation  of 
pain;  of  the  optic  nerve,  in  the  sensation  of  light;  of  a  motor  nerve,  in  con- 
traction of  the  muscle  to  which  it  is  distributed;  of  a  secretor  nerve,  in  the 
activity  of  the  related  gland,  etc.  It  is,  therefore,  evident  that  peculiarity  of 
nerve  function  depends  neither  upon  any  special  construction  or  activity  of 
the  nerve  itself,  nor  upon  the  nature  of  the  stimulus,  but  entirely  upon  the 
peculiarities  of  its  central  and  peripheral  end  organs. 

Nerve  stimuli  may  be  divided  into — 
i.  General  stimuli,  comprising  those  agents  which  are  capable  of  exciting  a 

nerve  in  any  part  of  its  course. 
2.  Special  stimuli,  comprising  those  agents  which  act  upon  nerves  only 

through  the  intermediation  of  the  end  organs. 

General  stimuli: 
i.  Mechanical:  as  from  a  blow,  pressure,  tension,  puncture,  etc. 

2.  Thermal:  heating  a  nerve  at  first  increases  and  then  decreases  its  excita- 
bility. 

3.  Chemic:  sensor  nerves  respond  somewhat  less  promptly  than  motor  nerves 
to  this  form  of  irritation. 

4.  Electric:  either  the  constant  or  interrupted  current. 

5.  The  normal  physiologic  stimulus: 

(a)  Centrifugal  or  efferent,  if  proceeding  from  the  center  toward  the 
periphery. 

(b)  Centripetal  or  afferent,  if  in  the  reverse  direction. 
Special  stimuli: 

1.  Light  or  ethereal  vibrations  acting  upon  the  end  organs  of  the  optic  nerve 
in  the  retina. 

2.  Sound  or  atmospheric  undulations  acting  upon  the  end  organs  of  the 
auditory  nerve. 

3.  Heat  or  vibrations  of  the  air  upon  the  end  organs  in  the  skin. 

4.  Chemic  agencies  acting  upon  the  end  organs  of  the  olfactory  and  gustatory 
nerves. 

Nature  of  the  Nerve  Impulse. — As  to  the  nature  of  the  nerve  impulse 
generated  by  any  of  the  foregoing  stimuli  either  general  or  special,  but  little 
is  known.     It  has  been  supposed  to  partake  of  the  nature  of  a  molecular 


PHYSIOLOGY    OF    NERVE   TISSUE.  75 

disturbance,  a  combination  of  physical  and  chemical  processes  attended  by 
the  liberation  of  energy,  which  propagates  itself  from  molecule  to  molecule. 
Judging  from  the  deflections  of  the  galvanometer  needle  it  is  probable  that 
when  the  nerve  impulse  makes  its  appearance  at  any  given  point  it  is  at  first 
feeble  but  soon  reaches  a  maximum  development  after  which  it  speedily 
declines  and  disappears.  It  may,  therefore,  be  graphically  represented  as  a 
wave-like  movement  with  a  definite  length  and  time  duration.  Under 
strictly  physiological  conditions  the  nerve  impulse  passes  in  one  direction 
only;  in  efferent  nerves  from  the  center  to  the  periphery,  in  afferent  nerves 
from  the  periphery  to  the  center.  Experimentally,  however,  it  can  be  dem- 
onstrated that  when  a  nerve  impulse  is  aroused  in  the  course  of  a  nerve  by 
an  adequate  stimulus  it  travels  equally  well  in  both  directions  from  the  point 
of  stimulation.  When  once  started  the  impulse  is  confined  to  the  single 
fiber  and  does  not  diffuse  itself  to  fibers  adjacent  to  it  in  the  same  nerve  trunk. 

Rapidity  of  Transmission  of  Nerve  Force. — The  passage  of  a  nervous 
impulse,  either  from  the  brain  to  the  periphery  or  in  the  reverse  direction, 
requires  an  appreciable  period  of  time.  The  velocity  with  which  the  impulse 
travels  in  human  sensor  nerves  has  been  estimated  at  about  190  feet  a  second, 
and  for  motor  nerves  at  from  100  to  200  feet  a  second.  The  rate  of  move- 
ment is,  however,  somewhat  modified  by  temperature,  cold  lessening  and 
heat  increasing  the  rapidity;  it  is  also  modified  by  electric  conditions,  by  the 
action  of  drugs,  the  strength  of  the  stimulus,  etc.  The  rate  of  transmission 
through  the  spinal  cord  is  considerably  slower  than  in  nerves,  the  average 
velocity  for  voluntary  motor  impulses  being  only  33  feet  a  second,  for  sensi- 
tive impressions  40  feet,  and  for  tactile  impressions  140  feet  a  second. 

Electric  Currents  in  Muscles  and  Nerves. — If  a  muscle  or  nerve  be 
divided  and  non-polarizable  electrodes  be  placed  upon  the  natural  longitu- 
dinal surface  at  the  equator,  and  upon  the  transverse  section,  electric  currents 
are  observed  with  the  aid  of  a  delicate  glavanometer.  The  direction  of 
the  current  is  always  from  the  positive  equatorial  surface  to  the  negative 
transverse  surface.  The  strength  of  the  current  increases  or  diminishes 
according  as  the  positive  electrode  is  moved  toward  or  from  the  equator. 
When  the  electrodes  are  placed  on  the  two  transverse  ends  of  a  nerve,  an 
axial  current  will  be  observed  the  direction  of  which  is  opposite  to  that  of 
the  normal  impulse  of  the  nerve. 

The  electromotive  force  of  the  strongest  nerve-current  has  been  estimated 
to  be  equal  to  the  0.026  of  a  Daniell  battery;  the  force  of  the  current  of  the 
frog  muscle,  about  0.05  to  0.08  of  a  Daniell. 

Negative  Variation  of  Currents  in  Muscles  and  Nerves. — If  a  muscle 
or  nerve  be  thrown  into  a  condition  of  tetanus,  it  will  be  observed  that  the 


76  HUMAN  PHYSIOLOGY. 

currents  undergo  a  diminution  of  negative  variation,  a  change  which  passes 
along  the  nerve  in  the  form  of  a  wave  and  with  a  velocity  equal  to  the  rate 
of  transmission  of  the  nerve  impulse.  The  wave-length  of  a  single  negative 
variation  has  been  estimated  to  be  eighteen  millimeters,  the  period  of  its 
duration  being  from  0.0005  to  °-ooo8  of  a  second. 

It  is  asserted  by  Hermann  that  perfectly  fresh,  uninjured  muscles  and 
nerves  are  devoid  of  currents,  and  that  the  currents  observed  are  the  result 
of  molecular  death  at  the  point  of  section,  this  point  becoming  negative  to 
the  equatorial  point.  He  applies  the  term  "action  currents"  to  the  currents 
obtained  when  a  muscle  is  thrown  into  a  state  of  activity. 

Electrotonus. — The  passage  of  a  direct  galvanic  current  through  a  portion 
of  a  nerve  excites  in  the  paits  beyond  the  electrodes  a  condition  of  electric 
tension,  or  electrotonus,  during  which  the  excitability  of  the  nerve  is  decreased 
near  the  anode  or  positive  pole,  and  increased  near  the  cathode  or  negative 
pole;  the  increase  of  excitability  in  the  catelectrotonic  area — that  nearest 
the  muscle — being  manifested  by  a  more  marked  contraction  of  the  muscle 
than  the  normal  when  the  nerve  is  irritated  in  this  region.  The  passage  of  an 
inverse  galvanic  current  excites  the  same  condition  of  electrotonus;  the 
diminution  of  excitability  near  the  anode,  the  anelectrotonic — that  now  nearest 
the  muscle — being  manifested  by  a  less  marked  contraction  than  the  normal 
when  the  nerve  is  stimulated  in  this  region.  Similar  conditions  exist  within 
the  electrodes.  Between  the  electrodes  is  a  neutral  point,  where  the  cate- 
lectrotonic area  merges  into  the  anelectrotonic  area.  If  the  current  be  a 
strong  one,  the  neutral  point  approaches  the  cathode;  if  weak,  it  approaches 
the  anode. 

When  a  nerve  impulse  passes  along  a  nerve,  the  only  appreciable  effect 
is  a  change  in  its  electric  condition,  there  being  no  change  in  its  temperature, 
chemic  composition,  or  physical  condition.  The  natural  nerve-currents, 
which  are  always  present  in  a  living  nerve  as  a  result  of  its  nutritive  activity, 
in  great  part  disappear  during  the  passage  of  an  impulse,  undergoing  a 
negative  variation. 

Law  of  Contraction. — If  a  feeble  galvanic  current  be  applied  to  a  recent 
and  excitable  nerve,  contraction  is  produced  in  the  muscles  only  upon  the 
making  of  the  circuit  with  both  the  direct  and  inverse  currents. 

If  the  current  be  moderate  in  intensity,  the  contraction  is  produced  in  the 
muscle,  both  upon  the  making  and  breaking  of  the  circuit,  with  both  the  direct 
and  inverse  currents. 

If  the  current  be  intense,  contraction  is  produced  only  when  the  circuit 
is  made  with  the  direct  current,  and  only  when  it  is  broken  with  the  inverse 
current. 


FOODS  AND    DIETETICS.  77 

FOODS  AND  DIETETICS. 

During  the  functional  activity  of  every  organ  and  tissue  of  the  body  the 
living  material  of  which  it  is  composed — the  protoplasm — undergoes  more  or 
less  disintegration.  Through  a  series  of  descending  chemic  stages  it  is 
reduced  to  a  number  of  simpler  compound,  which  are  of  no  further  value  to 
the  body,  and  which  are  in  consequence  eliminated  by  the  various  eliminat- 
ing or  excretory  organs — the  lungs,  kidneys,  skin,  liver.  Among  these  com- 
pounds the  more  important  are  carbon  dioxide,  urea,  and  uric  acid.  Many 
other  compounds,  inorganic  as  well  as  organic,  are  also  eliminated  in 
the  water  discharged  from  the  body,  in  which  they  are  held  in  solution. 
Coincident  with  this  disintegration  of  the  tissue  there  is  an  evolution  or 
disengagement  of  energy,  particularly  in  the  form  of  heat. 

In  order  that  the  tissues  may  regain  their  normal  composition  and  thus  be 
enabled  to  continue  in  the  performance  of  their  functions,  they  must  be  sup- 
plied with  the  same  nutritive  materials  of  which  their  protoplasm  originally 
consisted — viz.,  water,  inorganic  salts,  porteids,  sugar,  fat.  These  materials 
are  furnished  by  the  blood  during  its  passage  through  the  capillary  blood- 
vessels. The  blood  is  a  reservoir  of  nutritive  material  in  a  condition  to  be 
absorbed,  organized,  and  transformed  into  new  living  tissue. 

Inasmuch  as  the  loss  of  material  from  the  body  daily,  which  is  very  great, 
is  compensated  for  under  other  forms  by  the  blood,  it  is  evident  that  this 
fluid  would  rapidly  diminish  in  volume  were  it  not  restored  by  the  intro- 
duction of  new  and  corresponding  materials.  As  soon  as  the  blood  volume 
falls  to  a  certain  point,  the  sensations  of  hunger  and  thirst  arise,  which 
in  a  short  time  lead  to  the  necessity  of  taking  food. 

In  addition  to  the  direct  appropriation  of  food  by  the  tissues  it  is  highly 
probable  that  an  indefinite  amount  undergoes  oxidation  and  disintegration 
without  ever  becoming  an  integral  part  of  the  tissues,  and  thus  directly 
contributes  to  the  production  of  heat. 

Inanition  or  Starvation. — If  these  nutritive  principles  be  not  supplied 
in  sufficient  quantity,  or  if  they  are  withheld  entirely,  a  condition  of  physio- 
logic decay  is  established,  to  which  the  term  inanition  or  starvation  is  applied. 
The  phenomena  which  characterize  this  pathologic  process  are  as  follows — 
viz.,  hunger,  intense  thirst,  gastric  and  intestinal  uneasiness  and  pain, 
muscle  weakness  and  emaciation,  a  diminution  in  the  quantity  of  carbon 
dioxid  exhaled,  a  lessening  in  the  amount  of  urine  and  its  constituents  ex- 
creted, a  diminution  in  the  volume  of  the  blood,  an  exhalation  of  a  fetid 
odor  from  the  body,  vertigo,  stupor,  delirium,  and  at  times  convulsions,  a 
fall  of  bodily  temperature,  and,  finally,  death  from  exhaustion. 


78  HUMAN  PHYSIOLOGY. 

During  starvation  the  loss  of  different  tissues,  before  death  occurs,  averages 
jq,  or  40  per  cent.,  of  their  weight. 

Those  tissues  which  lose  more  than  40  per  cent,  are:  Fat,  93.3;  blood,  75; 
spleen,  71.4;  pancreas,  64.1;  liver,  52;  heart,  44.8;  intestines,  42.4;  muscle, 
42.3.  Those  which  lose  less  than  40  per  cent,  are:  The  muscular  coat  of 
the  stomach,  39.7;  pharynx  and  esophagus,  34.2;  skin,  S3-31  kidneys,  31.9; 
respiratory  apparatus,  22.2;  bones,  16.7;  eyes,  10;  nervous  system,  1.9. 

The  fat  entirely  disappears,  with  the  exception  of  a  small  quantity  which 
remains  in  the  posterior  portion  of  the  orbits  and  around  the  kidneys.  The 
blood  diminishes  in  volume  and  loses  its  nutritive  properties.  The  muscles 
undergo  a  marked  diminution  in  volume  and  become  soft  and  flabby.  The 
nervous  system  is  last  to  suffer,  not  more  than  two  per  cent.,  disappearing  be- 
fore death  occurs. 

The  appearances  presented  by  the  body  after  death  from  starvation  are  those 
of  anemia  and  great  emaciation;  almost  total  absence  of  fat;  bloodlessness; 
a  diminution  in  the  volume  of  the  organs;  an  empty  condition  of  the  stomach 
and  bowels,  the  coats  of  which  are  thin  and  transparent.  There  is  a  marked 
disposition  of  the  body  to  undergo  decomposition,  giving  rise  to  a  very  fetid 
odor. 

The  duration  of  life  after  a  complete  deprivation  of  food  varies  from  eight 
to  thirteen  days,  though  life  can  be  maintained  much  longer  if  a  quantity  of 
water  be  obtained.  The  water  is  more  essential  under  these  circumstances 
than  the  solid  matters,  which  can  be  supplied  by  the  organism  itself. 

The  different  alimentary  or  nutritive  principles  which  are  appropriated  by 
the  tissues,  and  which  are  contained  within  the  various  articles  of  food,  belong 
to  both  the  organic  and  inorganic  groups  and  chemic  compounds,  and  may  be 
classified  according  to  their  composition  as  follows: 

CLASSIFICATION  OF  ALIMENTARY  PRINCIPLES. 

1.  Protein  Group. — Nitrogenized,  C,  O,  H,  N,  S,  P. 

Principle.  Where  Found. 

Myosin Flesh  of  animals. 

Vitellin,  albumin       Yolk  of  egg,  white  of  egg. 

Fibrin,  globulin Blood  contained  in  meat.' 

Casein Milk,  cheese. 

Gluten Grain  of  wheat  and  other  cereals. 

Vegetable  albumin Soft,  growing  vegetables. 

Legumin Peas,  beans,  lentils,  etc. 

Gelatin Bones. 


FOODS  AND    DIETETICS.  79 

2.  Oleaginous  Group. — C,  O,  H. 

Animal  fats  and  oils. 1  Found  in  the  adipose  tissue  of  ani- 

Stearin,  olein \       mals,   seeds,   grains,   nuts,     fruits 

Palmitin,  fat  acids J       and  other  vegetable  tissues. 

3.  Carbohydrate  Group. — C,  O,  H. 
Saccharose,  or  cane-sugar Sugar-cane. 

Dextrose,  or  elucose      |  „     . 

t    ■  ?  Fruits. 

Levulose,  or  fruit-sugar J 

Lactose,  or  milk-sugar Milk. 

Maltoes      Malt,  malt  foods. 

Starch Cereals,    tuberous   roots,    and    legu- 
minous plants. 
Glycogen Liver,  muscles. 

4.  Inorganic  Group. — Water;  sodium  and  potassium  chlorids;  sodium 
calcium,  magnesium,  and  potassium  phosphates;  calcium  carbonate;  and 
iron. 

5.  Vegetable  Acid  Group. — Malic,  citric,  tartaric,  and  other  acids,  found 
principally  in  fruits. 

6.  Accessory  Foods. — Tea,  coffee,  alcohol,  cocoa,  etc. 

The  protein  principles  of  the  food,  after  undergoing  digestion  and  conver- 
sion into  amino  acids,  are  absorbed  and  transformed  into  the  form  of  pro- 
teins characteristic  of  the  blood  plasma  and  the  lymph.  Of  the  proteins 
thus  brought  into  relation  with  the  living  protoplasm,  a  small  percentage  only 
is  utilized  in  the  repair  of  its  substance.  This  is  known  as  tissue  protein.  A 
large  percentage  circulating  among  and  permeating  the  tissues  is  acted  upon 
by  them  directly,  and  reduced  to  simpler  compounds  without  ever  becoming  a 
part  of  the  tissue  itself.  This  is  known  as  circulating  protein.  In  the  process 
of  tissue  metabolism  all  the  proteins  suffer  disintegration,  and  give  rise  to 
the  production  of  some  carbon-holding  compound,  probably  fat,  and  some 
nitrogen-holding  compounds  which  eventually  produce  urea.  The  inter- 
mediate stages  are  possibly  represented  by  glycin,  creatin,  uric  acid,  etc.  An 
excess  of  proteins  in  the  food  is  followed  by  their  decomposition,  by  the  pan- 
creatic juice,  into  leucin  and  tyrosin,  which,  by  the  agency  of  the  liver,  are  con- 
verted into  urea.  The  disintegration  of  the  proteins  is  attended  by  the  dis- 
engagement of  heat:  they  thus  contribute  to  the  energy  of  the  body. 

The  oleaginous  principles,  after  digestion,  are  absorbed  into  the  blood, 
from  which  they  rapidly  disappear.  It  is  probable  that  a  portion  of  the  fat 
enters  directly  into  the  composition  of  living  protoplasm,  out  of  which  it  again 
emerges  at  some  subsequent  stage  in  the  form  of  small  drops  which  make 


80  HUMAN  PHYSIOLOGY. 

their  appearance  in  the  protoplasmic  cells  of  the  connective  areolar  tissue, 
thus  giving  rise  to  the  adipose  tissue.  Another  portion  probably  undergoes 
direct  oxidation. 

The  carbohydrate  principles,  after  digestion,  are  absorbed  as  dextrose  and 
temporarily  stored  up  in  the  liver  as  glycogen.  The  intermediate  stages 
which  sugar  passes  through  and  the  combination  into  which  it  enters  between 
its  absorbtion  and  its  elimination  are  but  imperfectly  understood.  That  it 
contributes  to  the  accumulation  of  fat  is  probable,  though  it  is  doubtful  if  it  is 
ever  converted  into  fat.  A  large  percentage  of  the  sugar  absorbed  is  at  once 
oxidized.  The  reduction  of  fat  and  sugar  to  carbon  dioxid  and  water,  under 
which  forms  they  are  eliminated  from  the  body,  is  accompanied  by  a  dis- 
engagement of  a  large  quantity  of  heat. 

Water  is  present  in  all  the  fluids  and  solids  of  the  body.  It  promotes  the 
absorbtion  of  new  material  from  the  alimentary  canal;  it  holds  the  various 
ingredients  of  the  blood,  lymph,  and  other  fluids  in  solution;  it  hastens  the 
absorption  of  waste  products  from  the  tissues,  and  promotes  their  speedy 
elimination  from  the  body. 

Sodium  chlorid  is  present  in  all  parts  of  the  body  to  the  extend  of  no  gm. 
The  average  amount  eliminated  daily  is  15  gm.  Its  necessity  as  an  article  of 
diet  is  at  once  apparent.  Taken  as  a  condiment,  it  imparts  sapidity  to  the 
food,  excites  the  flow  of  the  digestive  fluids,  promotes  the  absorption  and 
assimilation  of  the  albumins,  influences  the  passage  of  nutritive  material 
through  animal  membranes,  and  furnishes  the  chlorin  for  the  free  hydro- 
chloric acid  of  the  gastric  juice.  In  some  unknown  way  it  favorably  promotes 
the  activity  of  the  general  nutritive  process. 

The  potassium  salts  are  also  essential  to  the  normal  activity  of  the  nutri- 
tive process.  When  deprived  of  these  salts,  animals  become  weak  and  ema- 
ciated. When  given  in  small  doses,  they  increase  the  force  of  the  heart-beat, 
raise  the  arterial  pressure,  and  thus  increase  the  action  of  the  circulation  of  the 
blood. 

The  calcium  phosphate  and  carbonate  are  utilized  in  imparting  solidity  to  the 
tissues,  more  especially  the  bones  and  teeth.  Many  articles  of  food  contain 
these  salts  in  quantities  sufficient  to  restore  the  amount  lost  daily. 

The  vegetable  acids  increase  the  secretions  of  the  alimentary  canal,  and  are 
apt,  in  large  amounts,  to  produce  flatulence  and  diarrhea.  After  entering 
into  combination  with  bases  to  form  salts,  they  stimulate  the  action  of  the 
kidneys  and  promote  a  greater  elimination  of  all  the  urinary  constituents. 
In  come  unknown  way  they  influence  nutrition ;  when  deprived  of  these  acids, 
the  individual  becomes  scorbutic. 

The  accessory  foods,  coffee  and  tea,  when  taken  in  moderation,  overcome 
the  sense  of  fatigue  and  mental  unrest  consequent  on  excessive  physical  and 


FOODS  AND    DIETETICS.  8 1 

mental  exertion.  Coffee  increases  the  action  of  the  intestinal  glands  and  acts 
as  a  laxative.  After  absorption,  its  active  principle,  caffein,  stimulates  the 
action  of  the  heart,  raises  the  arterial  pressure,  and  excites  the  action  of  the 
brain.  Tea  acts  as  an  astringent,  owing  to  the  tannic  acid  it  contains.  One 
effect  of  the  tannic  acid  is  to  coagulate  the  digestive  ferments  and  to  interfere 
with  the  activity  of  the  digestive  process. 

Alcohol,  when  introduced  into  the  system  in  small  quantities,  undergoes 
oxidation  and  contributes  to  the  production  of  force,  and  is  thus  far  a  food. 
It  excites  the  gastric  glands  to  increased  secretion,  improves  the  digestion, 
accelerates  the  action  of  the  heart,  and  stimulates  the  activities  of  the  nerve 
centers.  In  zymotic  diseases,  and  in  all  cases  of  depression  of  the  vital 
powers,  it  is  most  useful  as  a  restorative  agent.  When  taken  in  excessive 
quantities,  it  is  eliminated  by  the  lungs  and  kidneys.  The  metamorphosis  of 
the  tissue  is  retarded,  the  elimination  of  urea  and  carbonic  acid  is  lessened, 
the  temperature  is  lowered,  the  muscular  powers  are  impaired,  and  the  resist- 
ance to  depressing  external  influences  is  diminished.  When  taken  through- 
out a  long  period  of  time,  alcohol  impairs  digestion,  produces  gastric  catarrh, 
and  disorders  the  secreting  power  of  the  hepatic  cells.  It  also  diminishes  the 
muscular  power  and  destroys  the  structure  and  composition  of  the  cells  of  the 
brain  and  spinal  cord.  The  connective  tissue  of  the  body  increases  in  amount, 
and,  subsequently  contracting,  gives  rise  to  sclerosis. 

A  proper  combination  of  various  alimentary  principles  is  essential  for 
healthy  nutrition,  no  one  class  being  capable  of  maintaining  life  for  any 
definite  length  of  time. 

The  protein  food  in  excess  promotes  the  arthritic  diathesis,  manifesting 
itself  as  gout,  gravel,  etc. 

The  oleaginous  food  in  excess  gives  rise  to  the  bilious  diathesis,  while  a 
deficiency  of  it  promotes  the  scrofulous. 

The  farinaceous  food  when  long  continued  in  excess,  favors  the  rheumatic 
diathesis  by  the  development  of  lactic  acid. 

The  quantities  of  the  different  nutritive  materials  which  are  required 
daily  for  the  growth  and  repair  of  the  tissues  and  for  the  evolution  of  heat 
have  been  variously  estimated  by  different  observers.  The  following  table 
shows  the  average  diet  scale  of  Vierordt,  and  the  amount  of  waste  products  to 
which  it  would  give  rise: 


82  HUMAN  PHYSIOLOGY. 

Comparison  of  the  Ingesta  and  Egesta. 

Ingesta.  Egesta. 

Protein 120  grams.       Urea 40  grams. 

Fat .  .        90  grams.       Inorganic  salts 32  grams. 

Starch 330  grams.       Feces 104  grams. 

Inorganic  salts 32  grams.       Carbon  dioxid 800  grams. 

Water 2,800  grams.       Water 3,096  grams. 

Oxygen 700  grams.  

Total 4,072  grams. 


Total 4,072  grams. 

Other  estimates  as  to  the  amount  of  the  organic  substances  required  daily 
are  as  follows: 

Ranke.  Voit.  Atwater.       Moleschott. 

Protein 100  118  125  130  grams. 

Fat 100  50  125  84  grams. 

Starch 240  500  400  404  grams. 

The  Energy  of  the  Animal  Body. — The  food  consumed  daily  not  only 
repairs  the  loss  of  material  from  the  body,  but  also  furnishes  the  energy  to  re- 
place that  which  is  expended  daily  in  the  shape  of  heat  and  motion.  All  the 
energy  of  the  body  can  be  traced  to  the  chemic  changes  going  on  in  the  tissues, 
and  more  particularly  to  those  changes  involved  in  the  oxidation  of  the  foods. 

The  amount  of  heat  yielded  by  any  given  food  principle  can  be  determined 
by  burning  it  to  carbon  dioxid  and  water,  and  ascertaining  the  extent  to 
which  it  will,  when  so  liberated,  raise  the  temperature  of  a  given  volume  of 
water.  This  amount  of  heat  may  be  expressed  in  calories.  A  calorie  is  the 
amount  of  heat  required  to  raise  the  temperature  of  one  kilogram  of  water  one 
degree  Centigrade. 

The  following  estimates  give,  approximately,  the  number  of  calories  pro- 
duced when  the  food  is  reduced  within  the  body  to  urea,  carbon  dioxid,  and 
water: 

1  gram  of  protein  yields  4,124  kilogram  calories. 
1  gram  of  fat  yields  9,353  kilogram  calories. 
1  gram  of  starch  yields    4,116  kilogram  calories. 

The  total  number  of  kilogram  calories  yielded  by  any  given  diet  scale  can 
be  readily  determined  by  multiplying  the  preceding  factors  by  the  quantities 


FOODS   AND    DIETETICS.  83 

of  material  consumed.     The  diet  scale  of  Ranke,  for  example,  yields  the 
following  amount: 

100  grams  of  protein  yield  412  .4  calories. 
100  grams  of  fat  yield  935 .3  calories. 

240  grams  of  starch  yield    987  .8  calories. 


Total 2,335  •  5  calories 

It  has  also  been  determined  experimentally  that  one  gram  of  proteid,  one 
gram  of  fat,  and  one  gram  of  starch,  when  completely  oxidized,  will  yield 
energy  sufficient  to  perform,  1,850,  3,841,  and  1,567  kilogrammeters  of  work, 
respectively.  A  kilogrammeter  of  work  is  one  kilogram  raised  one  meter 
high. 

The  total  energy  of  the  Ranke  diet  scale  can  be  easily  calculated — e.g., 

100  grams  of  proteid  yield  185,000  kilogrammeters. 
100  grams  of  fat  yield  384,100  kilogrammeters. 

240  grams  of  starch  yield    397,680  kilogrammeters. 


Total 966,780  kilogrammeters. 

It  will  be  thus  seen  that  the  food  consumed  daily  yields  2,335  kilogram 
calories,  which  can  be  translated  into  its  mechanical  equivalent,  966,780 
kilogrammeters  of  work. 

The  amount  of  food  required  in  twenty-four  hours  is  estimated  from  the 
total  quantity  of  carbon  and  nitrogen  excreted  from  the  body  in  twenty-four 
hours,  these  two  elements  representing  the  waste  or  destruction  of  the  carbon- 
aceous and  nitrogenized  compounds.  It  has  been  determined  by  experimen- 
tation that  about  4,600  grains  of  carbon  and  about  300  grains  of  nitrogen  are 
eliminated  from  the  body  daily,  the  ratio  being  about  15  to  1.  That  the  body 
may  be  kept  in  its  normal  condition,  a  proper  proportion  of  carbonaceous 
(bread)  to  nitrogenized  (meat)  food  should  be  observed  in  the  diet. 

The  method  of  determining  the  proper  amounts  of  both  kinds  of  food  is  as 
follows: 

1,000  grs.  of  bread  (2  oz.)  contain  300  grs.  C.  and  10  grs.  N. 

To  obtain  the  requisite  amount  of  nitrogen  from  bread,  30,000  grains,  or 
about  four  pounds,  containing  9,000  grains  of  carbon  and  300  of  nitrogen, 
would  have  to  be  consumed.  On  such  a  diet  there  would  be  a  large  excess  of 
carbon,  which  would  be  undesirable.     On  a  meat  diet  the  reverse  obtains: 

1,000  grs.  of  meat  (2  oz.)  contain  100  grs.  C.  and  30  grs.  N. 


84 


HUMAN  PHYSIOLOGY. 


To  obtain  the  requisite  amount  of  carbon  from  meat,  45,000  grains,  or 
about  6§  pounds,  containing  4,500  grains  of  carbon  and  1,350  grains  of 
nitrogen  would  have  to  be  consumed.  Under  such  circumstances  there 
would  arise  an  excess  of  nitrogen  in  the  system,  which  would  be  equally 
undesirable  and  injurious.  By  combining  these  two  articles,  however,  in 
proper  proportion,  the  requisite  amounts  of  carbon  and  nitrogen  can  be 
obtained  without  any  excess  of  either — e.  g. : 

2  pounds  of  bread  contain  4,630  grs.  C.  and  154  grs.  N. 
f  pounds  of  meat  contain       463  grs.  C.  and  154  grs.  N. 

5,093  C.  308  N. 

The  amount  of  carbon  and  nitrogen  necessary  to  compensate  for  the  loss 
to  the  system  daily  would  be  contained  in  the  foregoing  amount  of  food.  As 
about  3  \  ounces  of  oil  or  butter  are  consumed  daily,  the  quantity  of  bread 
can  be  reduced  to  19  ounces.  In  the  quantities  of  bread  and  meat  just  men- 
tioned there  are  4.2  ounces  albumin,  9.3  sugar  and  starch. 

The  alimentary  principles  are  not  introduced  into  the  body  as  such,  but 
are  combined  in  proper  proportions  to  form  compound  substances,  termed 
foods — e.  g.,  bread,  milk,  eggs,  meat,  etc. — the  nutritive  value  of  each  de- 
pending upon  the  extent  to  which  these  principles  exist. 

The  following  tables  show  the  average  composition  of  various  articles  of 
food: 

COMPOSITION  OF  ANIMAL  FOODS. 


In  100  parts. 

Beef 
(lean). 

Veal. 

Mutton. 

Pork. 

Fowl. 

Fish 
(Mack- 
erel). 

Water 

67  .00 

70.70 

62.80 

52.00 

63.70 

73-4Q 

Protein 

19.10 

19.70 

17.90 

16.20 

18.70 

18.10 

Fat 

12  .7 

7  30 

17.40 

28.6 

i5-5o 

6.70 

Carbohydrates  . . 

0.50 

0.80 

0.60 

0.60 

1 .20 

0.40 

Salts 

1.30 

0.80 

o-75 

0.80 

0.8 

0.90 

FOODS  AND   DIETETICS. 
COMPOSITION  OF  VEGETABLE  FOODS. 


85 


In  100  parts. 


Beans 

(white). 

Peas 

(dried). 

Potatoes 
(boiled). 

Turnips. 

Cab- 
bage. 

Aspara- 
gus 
(cooked). 


Water 

12  .60 

9-5o 

75-5o 

89.60 

91.50 

91 .60 

Protein 

15.80 

17-3 

1 .90 

1 .00 

1 .20 

1 .70 

Fat 

1 .60 

0.90 

0. 10 

0.20 

0.30 

7.  .00 

Carbohydrates  . . 

59-9o 

62.50 

20.00 

7.80 

5-5o 

2  .10 

Cellulose 

7-5o 

7.60 

1 .70 

0.80 

0.70 

1 .00 

Salts 

2.6 

2  .20 

0.80 

0.60 

0.80 

0.60 

COMPOSITION  OF  CEREAL  FOODS. 


In  100  parts. 

Wheat- 
flour. 

Rye- 
flour. 

Barley- 
pearls. 

Oat- 
meal. 

Corn- 
meal. 

Rice. 

Water 

11. 4          12.90 

11.50 

7.80 

12.50 

12.30 

Protein 

10.70 

5-3 

6.6 

13.40 

7-50 

6.50 

Fat 

1.70 

0.80 

1 .00 

6.60         1 .70 

0.30 

Carbohydrates. . . 

70.90 

76.90 

76.10 

65-2°       73-5o 

76  .90 

Cellulose 

4  SO 

3.60         4.00 

5.60       4.00       3.70 

Salts . 


0.80         0.5  0.80  1.40         0.80         0.30 


86  HUMAN  PHYSIOLOGY. 

DIGESTION. 

Digestion  is  a  physical  and  chemic  process  by  which  the  food  introduced 
into  the  alimentary  canal  is  liquefied  and  its  nutritive  principles  are  trans- 
formed by  the  digestive  fluids  into  new  substances  capable  of  being  absorbed 
into  the  blood. 

The  digestive  apparatus  consists  of  the  alimentary  canal  and  its  appen- 
dages— viz.,  teeth,  lips  and  tongue;  the  salivary,  gastric  and  intestinal  glands; 
the  liver  and  pancreas. 

Digestion  may  be  divided  into  several  stages;  prehension,  mouth  digestion 
(mastication  and  insalivation),  deglutition,  gastric  and  intestinal  digestion, 
and  defecation. 

Prehension,  the  act  of  conveying  food  into  the  mouth,  is  accomplished  by 
the  hands,  lips,  and  teeth. 

MASTICATION. 

Mastication  is  the  trituration  of  the  food,  and  is  accomplished  by  the 
teeth  and  lower  jaw  under  the  influence  of  muscular  contraction.  When 
thoroughly  divided,  the  food  presents  a  larger  surface  for  the  solvent  action 
of  the  digestive  fluids,  thus  aiding  the  general  process  of  digestion. 

The  teeth  are  thirty-two  in  number,  sixteen  in  each  jaw,  and  divided  into 
four  incisors  or  cutting  teeth,  two  canines,  four  bicuspids,  and  six  molars 
or  grinding  teeth;  each  tooth  consists  of  a  crown  covered  by  enamel,  a  neck, 
and  a  root  surrounded  by  the  crusta  petrosa  and  embedded  in  the  alveolar 
process;  a  section  through  a  tooth  shows  that  its  substance  is  made  of  dentine, 
in  the  center  of  which  is  the  pulp  cavity  containing  blood-vessels  and  nerves. 

The  lower  jaw  is  capable  of  making  a  downward  and  an  upward,  a  lateral 
and  an  anteroposterior  movement,  dependent  upon  the  constructon  of  the 
temporomaxillary  articulation. 

The  jaw  is  depressed  by  the  contraction  of  the  digastric,  geniohyoid, 
mylohyoid,  and  platysma  myoides  muscles;  elevated  by  the  temporal,  masseter, 
and  internal  pterygoid  muscles;  moved  laterally  by  the  alternate  contraction  of 
the  external  pterygoid  muscles;  moved  anteriorly  by  the  pterygoid,  and  poste- 
riorly by  the  united  actions  of  the  geniohyoid,  mylohyoid,  and  posterior  fibers 
of  the  temporal  muscles. 

The  food  is  kept  between  the  teeth  by  the  intrinsic  and  extrinsic  muscles 
of  the  tongue  from  within,  and  the  orbicularis  oris  and  buccinator  muscles 
from  without. 


DIGESTION.  87 

The  movements  of  mastication,  though  orginating  in  an  effort  of  the 
will  and  under  its  control,  are,  for  the  most  part,  of  an  automatic  or  reflex 
character,  taking  place  through  the  medulla  oblongata  and  induced  by  the 
presence  of  food  within  the  mouth.  The  nerves  and  nerve-centers  involved 
in  this  mechanism  are  shown  in  the  following  table: 

Nerve  Mechanism  of  Mastication. 

Afferent  Nerves.  Efferent  Nerves. 

1.  Lingual  branch  of  5th  pair.  1.  Third  branch  of  5th  pair. 

2.  Glossopharyngeal.  2.  Hypoglossal. 

3.  Facial. 

The  impressions  made  upon  the  terminal  filaments  of  the  afferent  nerves 
are  transmitted  to  the  medulla;  motor  impulses  are  here  generated  which  are 
transmitted  through  motor  nerves  to  the  muscles  involved  in  the  movements 
of  the  lower  jaw.  The  medulla  not  only  generates  motor  impulses,  but 
coordinates  them  in  such  a  manner  that  the  movements  of  mastication  may 
be  directed  toward  the  accomplishment  of  a  definite  purpose. 

INSALIVATION. 

Insalivation  is  the  incorporation  of  the  food  with  the  saliva  secreted  by 
the  parotid,  submaxillary,  and  sublingual  glands;  the  parotid  saliva,  thin  and 
watery,  is  poured  into  the  mouth  through  Steno's  duct;  the  submaxillary 
and  sublingual  salivas,  thick  and  viscid,  are  poured  into  the  mouth  through 
Wharton's  and  Bartholin's  ducts. 

In  their  minute  structure  the  salivary  glands  resemble  one  another.  They 
belong  to  the  racemose  variety,  and  consist  of  small  sacs  or  vesicles,  which 
are  the  terminal  expansions  of  the  smallest  salivary  ducts.  Each  vesicle  or 
acinus  consists  of  a  basement  membrane  surrounded  by  blood-vessels  and 
lined  with  epithelial  cells.  In  the  parotid  gland  the  lining  sells  are  granular 
and  nucleated ;  in  the  submaxillary  and  sublingual  glands  the  cells  are  large, 
clear,  and  contain  a  quantity  of  mucigen.  During  and  after  secretion  very 
remarkable  changes  take  place  in  the  cells  lining  the  acini,  which  are  in  some 
way  connected  with  the  essential  constituents  of  the  salivary  fluids. 

In  the  living  serous  gland — e.  g.,  parotid — during  rest,  the  secretory 
cells  lining  the  acini  of  the  gland  are  seen  to  be  filled  with  fine  granules, 
which  are  often  so  abundant  as  to  obscure  the  nucleus  and  enlarge  the  cells 
until  the  lumen  of  the  acinus  is  almost  obliterated.  (See  Fig.  n.)  When 
the  gland  begins  to  secrete  the  saliva,  the  granules  disappear  from  the  outer 
boundary  of  the  cells,  which  then  become  clear  and  distinct.  At  the  end  of 
the  secretory  activity  the  cells  have  been  freed  of  granules  and  have  become 


88 


HUMAN  PHYSIOLOGY. 


smaller  and  more  distinct  in  outline.  It  would  seem  that  the  granular  matter 
is  formed  in  the  cells  during  the  period  of  rest  and  discharged  into  the  ducts 
during  the  activity  of  the  gland. 

In  the  mucous  glands — e.  g.,  submaxillary  and  sublingual — -the   changes 
that  occur  in  the  cells  are  somewhat  different.     (See  Fig.  12.)     During  the 


Fig.  11. — Cells  of  the  Alveoli  of  a  Serous  or  Watery  Salivary  Gland. — 

(Langley.") 
A.  After  rest.      B.  After  a  short  period  of  activity.     C.  After  a  prolonged  period 

of  activity. 

intervals  of  digestion  the  cells  lining  the  gland  are  large,  clear,  and  highly 
refractive,  and  contain  a  large  quantity  of  mucigin.  After  secretion  has  taken 
place  the  cells  exhibit  a  marked  change.  The  mucigin  cells  have 
disappeared,    and    in    their    place  are   cells  which  are  small,   dark,  and 


Fig.  12. — Section  of  a  Mucous  Gland. — (Lavdowsky.) 
A.  In  a  state  of  rest.      B.  After  it  has  been  for  some  time  actively  secreting. 

composed  of  protoplasm.  It  would  appear  that  the  cells,  during  rest, 
elaborate  the  mucigin,  which  is  discharged  into  the  tubules  during  secretory 
activity,  to  become  part  of  the  secretion. 

Saliva  is  an  opalescent,  slightly  viscid,  alkaline  fluid,  having  a  specific 
gravity  of  1.005.     Microscopic  examination  reveals  the  presence  of  salivary 


DIGESTION.  89 

corpuscles  and  epithelial  cells.  Chemically  it  is  composed  of  water,  proteid 
matter,  a  ferment  (ptyalin),  and  inorganic  salts.  The  amount  secreted  in 
twenty  hours  is  about  2  J  lbs.     Its  function  is  twofold: 

1.  Physical. — Softens  and  moistens  the  food,  agglutinates  it,  and  facilitates 

swallowing. 

2.  Chemic. — Converts  starch  into  sugar.  This  action  is  due  to  the  presence 
of  an  enzyme,  ptyalin.  Its  power  of  converting  starch  into  sugar  is 
manifested  most  decidedly  at  the  temperature  of  the  living  body  and  in  a 
slightly  alkaline  medium.  The  conversion  of  starch  into  sugar  takes  place 
through  several  stages,  the  nature  of  which  depends  upon  the  structure  of 
the  starch  granule.  This  consists  of  two  portions,  a  stroma  of  cellulose 
and  a  contained  material,  granulose,  which  is  the  more  abundant  and  im- 
portant of  the  two.  When  subjected  to  the  action  of  boiling  water,  the 
starch  granule  swells  and  bursts,  forming  a  viscid,  opalescent  mass  of  starch 
paste.  If  saliva  be  now  added  to  this  paste  and  kept  at  a  temperature 
of  1040  F.  for  a  few  minutes,  the  paste  becomes  clear  and  limpid.  The 
first  stage  in  the  digestion  is  now  complete,  with  the  formation  of  soluble 
starch.  If  the  action  of  saliva  be  continued,  a  number  of  substances  in- 
termediate between  starch  and  sugar  are  formed,  to  which  the  name 
dextrin  has  been  given. 

(a)  Erythrodextrin,  which  gives  the  reddish-brown  color  with  iodin. 
As  the  digestion  continues  and  sugar  is  formed,  the  erythrodextrin 
disappears,  giving  way  to — 

(b)  A  chr 00 dextrin,  which  yields  no  coloration  with  iodin,  but  which  may 
be  precipitated  by  alcohol. 

The  sugar  formed  by  the  action  of  saliva  is  maltose  the  formula  for  which  is 
Cl2H22Ou.     A  small  quantity  of  dextrin  is  also  formed. 

The  successive  stages  in  the  conversion  of  starch  into  sugar  may  be  repre- 
sented by  the  following  schema: 

c..      ,  ,   ,  ,  ,  /  erythrodextrin  f  achroodextrin 

Starch  =  soluble  starch  =     <  ,  =    < 

^      maltose.  [      maltose. 

NERVE  MECHANISM  OF  INSALIVATION. 

Afferent  Nerves.  Efferent  Nerves. 

1.  Lingual  branch  of  5th  pair.  1.  Auriculotemporal   branch   of   5th 

2.  Taste  fibers  in  the  glosso-  pair,  for  parotid  gland. 

pharyngeal.  2.  Chorda    tympani,    for    submaxil- 

3.  Taste  fibers  in  the  laryand  sublingual  glands, 
chorda  tympani.                                   3.  Sympathetic  for  all  the  glands. 


90  HUMAN  PHYSIOLOGY. 

The  nerve  centers  exciting,  through  efferent  nerves  the  secretion  of  saliva 
are  located  in  the  medulla  oblongata  and  may  be  aroused  to  action  (i)  by 
nerve  impulses  descending  from  the  brain  in  consequence  of  psychic  states 
induced  by  the  sight  and  odor  of  food  and  (2)  by  nerve  impulses  reflected 
through  afferent  nerves  from  the  mouth  developed  by  the  taste  of  food.  The 
afferent  nerves  thus  stimulated  in  the  second  instance  are  those  stated  in 
the  foregoing  tabulation. 

That  the  efferent  nerves  in  the  same  tabulation  are  active  in  the  production 
of  the  secretion  is  shown  by  the  following  facts: 

Stimulation  of  the  auriculotemporal  branch  increases  the  flow  of  saliva 
from  the  parotid  gland;  division  arrests  it. 

Stimulation  of  the  chorda  tympani  is  followed  by  a  dilatation  of  the  blood- 
vessels of  the  submaxillary  and  sublingual  glands,  an  increased  flow  of  blood 
and  an  abundant  discharge  of  saliva;  division  of  the  nerve  arrests  the  secre- 
tion. 

Stimulation  of  the  cervical  sympathetic  is  followed  by  a  contraction  of  the 
blood-vessels,  a  diminished  flow  of  blood,  and  a  diminution  of  the  secretion, 
which  now  becomes  thick  and  viscid;  division  of  the  sympathetic  is  not, 
however,  followed  by  complete  dilatation  of  the  vessels.  There  is  evidence 
of  the  existence  of  a  local  vasomotor  mechanism,  which  is  inhibited  by  the 
chorda  tympani. 

DEGLUTITION. 

Deglutition  is  the  act  of  transferring  food  from  the  mouth  into  the  stomach, 
and  may  be  divided  into  three  stages: 

1.  The  passage  of  the  bolus  from  the  mouth  into  the  pharynx. 

2.  From  the  pharynx  into  the  esophagus. 

3.  From  the  esophagus  into  the  stomach. 

In  the  first  stage,  which  is  entirely  voluntary,  the  mouth  is  closed  and  res- 
piration momentarily  suspended;  the  tongue,  placed  against  the  roof  of  the 
mouth,  arches  upward  and  backward,  and  forces  the  bolus  into  the  fauces. 

In  the  second  stage,  which  is  entirely  reflex,  the  palate  is  made  tense  and 
directed  upward  and  backward  by  the  levatores  palati  and  tensores  palati 
muscles;  the  bolus  is  grasped  by  the  superior  constrictor  muscle  of  the  phar- 
ynx and  rapidly  forced  into  the  esophagus. 

The  food  is  prevented  from  entering  the  posterior  nares  by  the  uvula  and 
the  closure  of  the  posterior  half-arches  (the  palatopharyngeal  muscles) ;  from 
entering  the  larynx  by  its  ascent  under  the  base  of  the  tongue  and  the  action 
of  the  epiglottis. 

In  the  third  stage  the  longitudinal  and  circular  muscle-fibers,  contracting 
from  above  downward,  strip  the  bolus  into  the  stomach. 


DIGESTION.  91 

GASTRIC  DIGESTION. 

The  Stomach. — Immediately^  beyond  the  termination  of  the  esophagus 
the  alimentary  canal  expands  and  forms  a  receptacle  for  the  temporary  re- 
tention of  the  food.  To  this  dilatation  the  term  stomach  has  been  applied. 
This  organ  is  somewhat  pyriform  in  outline,  and  occupies  the  upper  part  of 
the  abdominal  cavity.  It  is  about  13  inches  long,  5  deep,  and  3^  wide,  and 
has  a  capacity  of  about  five  pints.  It  presents  two  orifices,  the  cardiac  or 
esophageal,  and  the  pyloric;  two  curvatures,  the  lesser  and  the  greater. 

The  left  or  cardiac  end  of  the  stomach  is  enlarged,  and  forms  the  fundus; 
the  right  end  is  much  narrower,  and  forms  the  pylorus.  The  stomach  pos- 
sesses three  coats: 

1.  The  serous,  or  reflection  of  the  peritoneum. 

2.  The  muscular,  the  fibers  of  which  are  arranged  in  a  longitudinal,  a  circular, 
and  an  oblique  direction.  As  the  pyloric  end  the  circular  fibers  increase  in 
number  and  form  a  thick  ring  or  band,  which  is  known  as  the  sphincter  of 
the  pylorus. 

3.  The  mucous,  which  is  somewhat  larger  than  the  muscular  coat,  and  in 
consequence  is  thrown  into  folds  or  rugae.  The  surface  of  the  mucous  coat 
is  covered  by  tall,  narrow,  columnar  epithelium. 

Gastric  Juice. — During  the  period  of  time  the  food  remains  in  the  stomach 
it  is  subjected  to  the  disintegrating  action  of  an  acid  fluid,  the  gastric  juice. 
This  fluid,  secreted  from  glands  in  the  mucous  membrane,  is  thoroughly 
incorporated  with  the  food  in  consequence  of  the  contractions  of  the  muscular 
coat.  The  food  is  gradually  liquefied  and  reduced  to  a  form  which  partly 
fits  it  for  passage  into  the  small  intestine  and  for  absorption  into  the  blood. 
Gastric  juice,  when  obtained  in  a  pure  state,  is  a  clear,  colorless  fluid,  decid- 
edly acid  in  reaction,  with  a  specific  gravity  of  1005.  It  is  composed  of  the 
following  ingredients: 

COMPOSITION  OF  GASTRIC  JUICE. 

Water 994.404 

Hydrochloric  acid 2.000 

Organic  matter 3-I05 

Inorganic  salts      2.201 

1000.000 

The  water  forms  by  far  the  largest  part  of  this  fluid,  and  serves  the  purpose 
of  holding  the  other  ingredients  in  solution,  and  by  its  saturating  power 
brings  them  into  relation  with  the  constituents  of  the  food.  Of  the  inorganic 
salts  the  sodium  and  potassium  chlorids  are  the  most  abundant  and  important. 


92  HUMAN  PHYSIOLOGY. 

The  hydrochloric  acid,  which  exists  in  a  free  state,  is  present  in  variable 
amounts.  In  the  foregoing  table  the  number  of  parts  a  thousand  is  much 
smaller  than  is  usually  stated.  According  to  most  observers,  hydrochloric 
acid  is  present  to  the  extent  of  from  0.2  to  0.3  part  a  hundred.  Though 
secreted  as  soon  as  the  food  enters  the  stomach,  the  acid  can  not  be  detected 
in  the  free  state  until  after  the  lapse  of  from  thirty  to  forty  minutes.  It  acidu- 
lates the  food  and  prevents  fermentative  changes. 

The  pepsin,  which  is  present  in  gastric  juice  associated  with  the  organic 
matter,  is  a  hydrolytic  ferment  or  enzyme.  When  freed  from  its  associations 
and  obtained  in  a  pure  state,  pepsin  presents  the  characteristics  of  a  colloid 
body.  It  has  the  power,  when  brought  into  relation  with  acidulated  proteins, 
of  transforming  them  into  peptones. 

Rennin. — In  addition  to  pepsin  a  second  ferment  exists  in  the  gastric  juice, 
to  which  the  term  rennin  has  been  given.  It  possesses  the  power  of  coagulat- 
ing the  caseinogen  of  milk.  It  exists  in  the  mucous  membrane,  from  which 
it  can  be  extracted  by  appropriate  means.  When  rennin  acts  on  caseinogen, 
the  latter  is  split  into  insoluble  casein  and  a  soluble  albumin.  Calcium  phos- 
phate is  essential  to  the  action  of  this  enzyme. 

Gastric  Glands. — Embedded  within  the  mucous  membrane  are  to  be 
found  enormous  numbers  of  tubular  glands,  which  though  resembling  one 
another  in  general  form,  differ  in  their  histologic  details  in  various  portions  of 
the  stomach. 

In  the  cardiac  end  or  fundus,  the  glands  consist  of  several  long  tubules, 
opening  into  a  short,  common  duct,  which  opens  by  a  wide  mouth  on  the 
surface  of  the  mucous  membrane.  Each  gland  consists  primarily  of  a  base- 
ment membrane  lined  by  epithelial  cells.  In  the  duct  the  epithelium  is  of  the 
columnar  variety,  resembling  that  covering  the  surface  of  the  mucous  mem- 
brane. The  secretory  portion  of  the  tubule  is  lined  by  a  layer  of  short,  poly- 
hedral, granular,  and  nucleated  cells,  which,  as  they  border  the  lumen  of  the 
tubule,  and  thus  occupy  the  central  portion  of  the  gland,  are  termed  central 
cells.  At  irregular  intervals,  between  the  central  cells  and  the  wall  of  the 
tubule,  are  found  large  oval,  reticulated  cells,  which,  on  account  of  their 
position,  are  termed  parietal  cells.     (See  Fig.  13.) 

Each  parietal  cell  is  in  relation  with  a  system  of  fine  canals,  which  open 
directly  into  the  lumen  of  the  gland.  It  is  estimated  that  the  fundus  contains 
about  five  million  glands.  In  the  pyloric  end  of  the  stomach  the  glands  are 
generally  branched  at  their  lower  extremities,  and  the  common  duct  is  long 
and  wide.  The  duct  is  lined  by  columnar  epithelium,  while  the  secreting 
part  is  lined  by  short,  slightly  columnar,  granular  cells.  The  parietal  cells 
are  entirely  wanting.     The  epithelium  covering  the  surface  of  the  mucous 


DIGESTION. 


93 


membrane  is  tall,  narrow  and  cylindric  in  shape,  and  consists  of  mucus- 
secreting  goblet  cells.  The  outer  half  of  the  cell  contains  a  substance,  mu- 
cinogen,  which  produces  mucin.  The  gastric  glands  in  both  situations  are  sur- 
rounded by  a  fine  connective  tissue,  which  supports  blood-vessels,  nerves, 
and  lymphatics. 

Changes  in  the  Cells  During  Secretion. — During  the  periods  of  rest  and 
secretory  activity  the  cells  of  the  glands  undergo  changes  in  structure  which 


Fig.  13. 

Diagram  showing  the  relation  of  the  ultimate  twigs  of  the  blood-vessels,  V.  and 
A,  and  of  the  absorbent  radicles  to  the  glands  of  the  stomach  and  the  different  kinds 
of  epithelium — viz.,  above  cylindric  cells;  small,  pale  cells  in  the  lumen,  outside 
which  are  the  dark  ovoid  cells. — (Yeo's  "  Text-book  of  Physiology.") 


are  supposed  to  be  connected  with  the  production  of  the  pepsin  and  hydro- 
chloric acid.  During  rest,  the  protoplasm  of  the  central  cells  becomes  filled 
with  granular  matter;  during  the  time  of  secretion  this  disappears,  presumably 
passing  into  the  lumen  of  the  tubule,  and  as  a  result  the  protoplasm  becomes 
clear  and  hyalin  in  appearance.  The  granular  material  is  probably  the 
mother  substance,  pepsinogen,  which,  inactive  in  itself,  yields  the  active  fer- 
ment, pepsin.  The  parietal  cells  during  digestion  increase  in  size,  but  do  not 
become  granular.     It  is  at  this  period  that  they  secrete  the  hydrochloric  acid. 


94  HUMAN  PHYSIOLOGY. 

After  digestion  they  rapidly  diminish  in  size  and  return  to  their  former  condi- 
tion.    The  pyloric  glands  secrete  pepsin  only. 

Mechanism  of  Secretion. — In  the  intervals  of  digestion,  the  mucous 
membrane  of  the  stomach  is  covered  with  a  layer  of  mucus."  As  soon  as  the 
food  passes  from  the  esophagus  into  the  stomach,  the  blood-vessels  dilate,  the 
circulation  becomes  more  active,  and  the  membrane  assumes  a  bright  red 
appearance.  Coincidentally,  small  drops  of  gastric  juice  begin  to  exude  from 
the  glands,  which  as  they  increase  in  number,  run  together  and  trickle  down 
the  sides  of  the  stomach.  This  pouring  out  of  fluid  continues  during  the 
presence  of  food  in  the  stomach. 

The  secretion  of  gastric  juice  is  mediated  by  nerve  centers  in  the  medulla 
oblongata.  From  these  centers,  nerve  impulses  descend  the  vagus  and 
vaso-motor  nerves  to  the  blood  vessels  and  epithelium  of  the  gastric  gland 
and  excite  them  to  action. 

The  nerve  centers  in  the  medulla  are  aroused  to  action  primarily  by  nerve 
impulses  descending  from  the  brain  in  consequence  of  psychic  states  devel- 
oped by  the  sight  and  odor  of  food  and  secondarily  by  nerve  impulses  reflected 
from  the  mouth  during  the  act  of  mastication. 

After  a  meal  has  been  swallowed  the  continued  secretion  of  gastric  juice  is 
attributed  to  the  development  in  the  pyloric  end  of  the  stomach,  by  the  action 
of  certain  articles  of  food,  e.  g.,  dextrin,  meat  broths,  soups,  or  by  the  first 
products  of  digestion,  some  chemic  agent  that  is  absorbed  into  the  blood  and 
in  due  time  reaches  the  gland  cells  and  stimulates  them.  Such  an  agent  is 
termed  secretin. 

Chemic  Action  of  the  Gastric  Juice. — By  the  combined  influence  of  the 
contraction  of  the  muscular  walls,  the  action  of  the  gastric  juice,  and  the 
temperature,  the  food  is  reduced  to  a  semiliquid  condition  and  acquires  a 
distinct  acid  odor.  This  semifluid  mass  will  vary  in  composition  and  appear- 
ance according  to  the  nature  of  the  food.  To  this  matter  the  term  chyme  has 
been  given. 

Meat  is  rapidly  disintegrated  by  the  solution  of  its  connective  tissue.  The 
fibers  thus  separated  are  readily  broken  up  into  particles  by  solution  of  the 
sarcolemma.  Well-cooked  meat  is  more  easily  digested,  owing  to  the  con- 
version of  the  connective  tissue  into  gelatin. 

Connective  tissues  in  the  raw  or  imperfectly  gelatinized  condition  are  very 
slowly  dissolved.  Cartilage,  tendons,  and  even  bones  will  in  time  be  corroded 
and  liquefied. 

Vegetables  are  not  easily  digested  unless  thoroughly  prepared  by  sufficient 
cooking.  The  nutritive  principles  are  inclosed  by  cellulose  walls,  which  are 
not  affected  by  gastric  juice.     The  influence  of  heat  and  moisture  softens 


DIGESTION.  95 

and  ruptures  the  cellulose  walls  so  as  to  permit  the  introduction  of  gastric 
juice  and  the  solution  of  its  nutritive  principles. 

The  principal  action  of  the  gastric  juice,  however,  is  the  transformation 
of  the  different  protein  principles  of  the  food  into  peptones,  the  intermediate 
stages  of  which  are  due  to  the  influence  of  the  acid  and  pepsin  respectively. 
As  soon  as  any  one  of  the  proteins  is  penetrated  by  the  acid  it  is  converted  into 
acid-protein,  a  fact  which  indicates  that  the  first  step  in  gastric  digestion  is  the 
acidification  of  the  proteins.  This  having  been  accomplished,  the  pepsin 
becomes  operative  and  in  a  varying  length  of  time  transforms  the  acid-protein 
into  a  new  form  of  protein  termed  peptone.  In  this  transformation  it  is 
possible  to  isolate  intermediate  bodies  by  the  addition  of  ammonium  and 
magnesium  sulphates,  to  which  the  term  proteoses  has  been  given.  Because 
of  the  order  in  which  they  are  obtained  they  have  been  divided  into  primary 
and  secondary.  This  supposed  change  is  represented  by  the  following 
scheme: 

Protein 

I 

Acid-protein 

I 

Proteose  (primary) 

I 

Proteose  (secondary) 

Peptone 

Peptones. — Peptones  are  the  final  products  of  the  digestion  of  protein 
bodies  in  the  stomach  and  differ  from  the  bodies  from  which  they  are  derived 
in  the  following  particulars: 

i.  They  are  diffusible — i.  e.,  capable  of  passing  readily  through  animal 
membranes. 

2.  They  are  soluble  in  water  and  in  saline  solution. 

3.  They  are  non-coagulable  by  heat  and  nitric  or  acetic  acids.  They  can 
be  readily  precipitated,  however,  by  tannic  acid,  by  bile  acids,  and  by 
mercuric  chlorid. 

The  duration  of  gastric  digestion  will  depend  largely  upon  the  quantity 
and  quality  of  the  food.  The  digestion  of  the  average  meal  occupies  from 
three  to  five  hours. 

Movements  of  the  Stomach. — During  the  period  of  gastric  digestion  the 
walls  of  the  stomach  become  the  seat  of  a  series  of  movements,  somewhat 


g6  HUMAN  PHYSIOLOGY. 

peristaltic  in  character,  which  serve  not  only  to  incorporate  the  gastric  juice 
with  the  food,  but  also  serve  to  eject  the  liquefied  portions  of  the  food  into 
the  intestine. 

After  the  entrance  of  the  food  both  the  cardiac  and  pyloric  orifices  are 
closed  by  the  contraction  of  their  sphincters.  Within  five  minutes  (in  the 
cat)  annular  constrictions  begin  in  the  pyloric  region  which  move  peristaltic- 
ally  towards  the  pylorus.  As  digestion  proceeds  these  constrictions  or  con- 
tractions become  more  frequent  and  more  vigorous.  The  result  is  a  tritura- 
tion and  liquefaction  of  the  food.  So  soon  as  it  is  liquefied  the  pylorus 
relaxes  and  permits  of  its  discharge  into  the  intestine.  The  pylorus  then 
closes  and  further  preparation  of  food  goes  on.  From  time  to  time  the  py- 
lorus relaxes  to  permit  the  discharge  of  prepared  and  liquefied  food  until 
digestion  is  completed.  The  reason  assigned  for  the  relaxation  of  the 
sphincter  muscle  is  the  presence  of  a  sufficient  amount  of  free  hydrochloric 
acid  on  the  gastric  side.  The  reason  assigned  for  its  contraction  after  the 
discharge  of  food  into  the  duodenum  is  the  presence  of  the  hydrochloric  acid 
in  this  region.  With  its  neutralization  by  the  alkalies  there  present  its  influence 
in  causing  contraction  of  the  sphincter  gradually  diminishes.  In  the  cardiac 
region  there  is  an  absence  of  peristalsis  though  the  muscle  wall  is  in  a  state 
of  active  tone.  The  fundus  acts  as  a  reservoir  for  food  and  delivers  its 
contents  to  the  pyloric  region  as  rapidly  as  it  is  ready  to  receive  them. 

TABLE    SHOWING   DIGESTIBILITY    OP   VARIOUS   ARTICLES    OF    FOOD. 

Hours.     Minutes. 

Eggs,  whipped i           20 

Eggs,  soft-boiled 3 

Eggs,  hard-boiled : 3           30 

Oysters,  raw 2           55 

Oysters,  stewed 3           30 

Lamb,  broiled 2           30 

Veal,  broiled 4 

Pork,  roasted 5           15 

Beefsteak,  broiled 3 

Turkey,  roasted 2           25 

Chicken,  boiled ...  .  .  4 

Chicken,  fricasseed 2           45 

Duck,  roasted 4 

Soup,  barley,  boiled 1           30 

Soup,  bean,  boiled 3 

Soup,  chicken,  boiled 3 

Soup,  mutton,  boiled 3           30 


urs 

Minutes, 

2 

30 

3 

20 

3 

45 

2 

30 

2 

30 

3 

30 

4 

3° 

3 

3° 

3 

45 

2 

3° 

DIGESTION.  97 

TABLE  SHOWING  DIGESTIBILITY  OF  VARIOUS  ARTICLES  OF  FOOD. — Continued. 

Liver,  beef,  broiled 

Sausage  broiled 

Green  corn,  boiled 

Beans,  boiled 

Potatoes,  roasted 

Potatoes,  boiled 

Cabbage,  boiled 

Turnips,  boiled 

Beets,  boiled 

Parsnips,  boiled 

INTESTINAL  DIGESTION. 

The  process  of  digestion  as  it  takes  place  in  the  small  intestine  is  exceed- 
ingly important  and  complex,  and  is  brought  about  by  the  action  of  the  pan- 
creatic juice,  the  bile,  and  the  intestinal  juice. 

The  contents  of  the  stomach  at  the  close  of  gastric  digestion  consist  of 
water,  inorganic  salts,  peptones,  undigested  proteins  and  starches,  maltose, 
cane-sugar,  liquefied  fats,  cellulose,  and  the  indigestible  portions  of  meats, 
cereals,  fruits,  etc.  This  so-called  chyme  is  quite  acid  in  reaction,  and  upon 
its  passage  through  the  now  open  pylorus  into  the  intestine  it  excites  a  reflex 
stimulation  and  secretion  of  the  intestinal  fluids,  which  are  decidedly  alkaline 
in  reaction,  and  which  have  a  neutralizing  action  on  the  chyme.  As  soon  as 
the  latter  becomes  alkaline  and  gastric  digestion  is  arrested,  the  various 
phases  of  intestinal  digestion  begin  which  eventuate  in  the  transformation 
of  all  the  remaining  undigested  nutritive  materials  into  absorbable  and 
assimilable  compounds. 

The  small  intestine  is  about  22  feet  in  length  and  about  i£  inches  in 
diameter.     Like  the  stomach,  it  possesses  three  coats,  as  follows: 

1.  The  serous,  or  peritoneal. 

2.  The  muscular,  the  fibers  of  which  are  arranged  for  the  most  part  circularly 
Some  of  the  fibers  are  so  arranged  as  to  form  longitudinal  bands. 

3.  The  mucous,  which  presents  a  series  of  transverse  folds,  known  as  the 
valvules  conniventes. 

Intestinal  Glands. — In  that  portion  of  the  small  intestine  known  as  the 
duodenum  are  to  be  found  a  number  of  small,  branched,  tubular  glands  (Brun- 
ner's),  the  acini  of  which  are  lined  by  short,  cylindric  cells,  similar  to  those 
lining  the  pyloric  glands.     From  the  duodenum  to  the  termination  of  the 

7 


98 


HUMAN  PHYSIOLOGY. 


intestine  the  mucous  membrane  contains  an  enormous  number  of  tubular 
glands  (Lieberkuhn's),  formed  by  an  inversion  of  the  basement  membrane 
and  lined  by  epithelial  cells.  The  common  secretion  of  these  intestinal 
glands  forms  the  intestinal  juice.  This  is  a  thin,  opalescent,  slightly  yellow- 
ish fluid,  alkaline  in  reaction,  and  contains  water,  salts  and  proteid  matter. 

The  function  of  the  intestinal  juice  is  but  incompletely  known.  It  appears 
to  have  the  power  of  converting  starch  into  dextrose.  It  is  doubtful  whether 
it  is  capable  of  digesting  either  proteins  or  fats.  Its  most  distinctive  action  is 
the  inversion  of  saccharose  into  dextrose  and  levulose,  maltose  into  dextrose, 
and  lactose  into  dextrose  and  galactose,  thus  preparing  them  for  absorption. 
This  change  is  dependent  on  the  presence  of  three  ferment  bodies  known 
as  saccharase,  maltase  and  lactase. 

By  reason  of  the  presence  of  the  enzyme  erepsin,  the  peptones,  the  final 
products  of  the  digestion  of  proteins  by  the  gastric  and  pancreatic  juices, 
are  still  further  reduced  to  the  condition  of  amino  acids. 

The  pancreatic  juice  is  secreted  by  the  pancreas,  a  flattened  gland,  about 
six  inches  long,  running  transversely  across  the  posterior  wall  of  the  abdomen 
behind  the  stomach;  its  duct  opens  into  the  duodenum. 


A  * 

Pig.  14. — One  Saccule  of  the  Pancreas  of  the  Rabbit  in  Different  States  of 
Activity. — (After  Kuhne  and  Lea.) 
A.  After  a  period  of  rest,  in  which  case  the  outlines  of  the  cells  are  indistinct  and 
the  inner  zone — i.e.,  the  part  of  the  cells  (a)  next  the  lumen  (c) — is  broad  and  filled 
with  fine  granules.  B.  After  the  gland  has  poured  out  its  secretion,  when  the  cell 
outlined  (d)  are  clearer,  the  granular  zone  (a)  is  smaller,  and  the  clear  outer  zone  is 
wider. 


The  pancreas  is  similar  in  structure  to  the  salivary  glands,  and  consists 
of  the  system  of  ducts  terminating  in  acini.  The  acini  are  tubular  or  flask- 
shaped,  and  consist  of  a  basement  membrane  lined  by  a  layer  of  cylindric, 
conic  cells,  which  encroach  upon  the  lumen  of  the  acini.  The  cells  exhibit 
a  difference  in  their  structure  (Fig.  14),  and  may  be  said  to  consist  of  two 


DIGESTION.  99 

zones — viz.,  an  outer  parietal  zone,  which  is  transparent  and  apparently  homo- 
geneous, staining  rapidly  with  carmin;  an  inner  zone,  which  borders  the  lumen, 
and  is  distinctly  granular  and  stains  but  slightly  with  carmin.  These  cells 
undergo  changes  similar  to  those  exhibited  by  the  cells  of  the  salivary  glands 
during  and  after  active  secretion.  As  soon  as  the  secretory  activity  of  the 
pancreas  is  established,  the  granules  disappear,  and  the  inner  granular  layer 
becomes  reduced  to  a  very  narrow  border,  while  the  outer  zone  increases 
in  size  and  occupies  nearly  the  entire  cell.  During  the  intervals  of  secre- 
tion, however,  the  granular  layer  reappears  and  increases  in  size  until  the 
outer  zone  is  reduced  to  a  minimum.  It  would  seem  that  the  granular 
matter  is  formed  by  the  nutritive  processes  occurring  in  the  gland  during 
rest,  and  is  discharged  during  secretory  activity7  into  the  ducts,  and  takes  part 
in  the  formation  of  the  pancreatic  secretion. 

Towards  the  outer  extremity  of  the  pancreas  there  are  found  among  the 
acini  collections  of  globular  cells  arranged  in  rods  or  columns  separated  by 
connective  tissue.  They  have  been  termed  after  their  discoverer,  the  Islands 
of  Langerhans.  It  is  believed  they  produce  an  internal  secretion  which 
in  some  way  regulates  sugar  metabolism. 

The  pancreatic  juice  is  transparent,  colorless,  strongly  alkaline,  and  viscid, 
and  has  a  specific  gravity  of  1040.  It  is  one  of  the  most  important  of  the 
digestive  fluids,  as  it  exerts  a  transforming  influence  upon  all  classes  of 
alimentary  principles,  and  has  been  shown  to  contain  at  least  three  distinct 
ferments.     It  has  the  following  composition: 

COMPOSITION    OF    PANCREATIC   JUICE. 

Water 900 .76 

Protein  material 90 .  44 

Inorganic  salts 8 .  80 

1,000.00 

The  pancreatic  juice  is  characterized  by  its  action: 

1.  Upon  starch.  When  starch  is  subjected  to  the  action  of  the  juice,  it  is 
at  once  transformed  into  maltose;  the  change  takes  place  more  rapidly 
than  when  saliva  is  added.  This  action  is  caused  by  the  presence  of  a 
special  ferment,  amylopsin. 

2.  Upon  protein.  The  protein  bodies  which  escape  digestion  in  the  stomach 
are  converted  into  peptones  by  the  action  of  the  alkali  and  ferment. 
The  first  effect  of  the  alkali  is  to  change  the  protein  into  an  alkali-protein, 
a  fact  which  indicates  that  in  the  digestion  of  protein  by  pancreatic  juice, 
the  first  stage  is  alkalinization.  This  having  been  accomplished,  the  fer- 
ment trypsin  transforms  the  alkali-albumin  into  peptone.     For  the  same 


100  HUMAN  PHYSIOLOGY. 

reasons  it  is  believed  that  here  also  these  bodies  are  preceded  in  their 
development  by  proteoses,  of  which  there  is  probably  but  one  form,  viz., 
secondary  proteoses.  Long- continued  action  of  the  pancreatic  juice,  as 
previously  stated,  decomposes  the  peptone  into  leucin,  tyrosin,  etc. 
3 .  Upon/ate.  The  most  striking  action  of  the  pancreatic  juice  is  the  cleavage 
of  the  neutral  fats  into  a  fat  acid  and  glycerine  with  an  accompanying 
emulsification  of  the  remaining  neutral  fat. 

When  pancreatic  juice  comes  into  relation  with  neutral  fats  they  at  once 
combine  with  water  after  which  they  undergo  a  cleavage  into  a  fat  acid, 
oleic,  palmitic  and  stearic  and  glycerin.  The  fat  acids  then  combine 
with  alkalies  and  form  soaps.  Coincident  with  this  a  portion  of  the  fat 
is  finally  divided  and  held  in  suspension  in  the  soap  forming  an  emulsion. 
This  emulsified  fat  subsequently  also  undergoes  the  customary  cleavage 
into  fat  acids  and  glycerin.     This  is  accomplished  by  the  enzyme  steapsin. 

The  bile  has  an  important  function  in  the  elaboration  of  the  food  and  in 
its  preparation  for  absorption.  It  is  a  golden-brown,  viscid  fluid,  having  a 
neutral  or  alkaline  reaction  and  a  specific  gravity  of  1020. 

COMPOSITION   OF  BILE. 

Water , !  859 . 2 

Sodium  glycocholate     1 

Sodium  taurocholate    J  9-4 

Fat 9.2 

Cholesterin 2.6 

Mucus  and  coloring-matter 29 .8 

Salts 7.8 


1,000.0 


The  biliary  salts,  sodium  glycocholate  and  taurocholate,  are  characteristic 
ingredients,  and  by  the  process  of  secretion  are  formed  in  the  liver  from 
materials  furnished  by  the  blood.  It  is  probable  that  they  are  derived  from 
the  nitrogenized  compounds,  though  the  stages  in  the  process  are  unknown. 
They  are  reabsorbed  from  the  small  intestine  to  play  some  ulterior  part  in 
nutrition. 

Cholesterin  is  a  product  of  waste  taken  up  by  the  blood  from  the  nerve 
tissues  and  excreted  by  the  liver.  It  crystallizes  in  the  form  of  rhombic  plates 
which  are  quite  transparent.  When  retained  within  the  blood,  it  gives  rise 
to  the  condition  of  cholesteremia,  attended  with  severe  nervous  symptoms. 
It  is  given  off  in  the  feces  under  the  form  of  stercorin. 


DIGESTION.  IOI 

The  coloring-matters  which  give  the  tints  to  the  bile  are  biliverdin  and 
bilirubin,  and  are  probably  derived  from  the  coloring-matter  of  the  blood. 
Their  presence  in  any  fluid  can  be  recognized  by  adding  to  it  nitric  acid  con- 
taining nitrous  acid,  when  a  play  of  colors  is  observed,  beginning  with  green, 
blue,  violet,  red  and  yellow. 

The  bile  is  both  a  secretion  and  an  excretion;  it  is  constantly  being  formed 
and  discharged  by  the  hepatic  ducts  into  the  gall-bladder,  in  which  it  is  stored 
up  during  the  intervals  of  digestion.  As  soon  as  food  enters  the  intestines  it  is 
poured  out  abundantly  by  the  contraction  of  the  walls  of  the  gall-bladder. 

The  amount  secreted  in  twenty-four  hours  is  about  2\  pounds. 

Functions  of  the  Bile  : 

i.  It  assists  in  the  digestion  of  the  fats  and  promotes  their  absorption. 

2.  It  tends  to  prevent  putrefactive  changes  in  the  food. 

3.  It  stimulates  the  secretion  of  the  intestinal  glands,  and  excites  the  normal 
peristaltic  movement  of  the  bowels. 

The  digested  food,  the  chyme,  is  a  grayish,  pultaceous  mass,  but  as  it  passes 
through  the  intestines  it  becomes  yellow  from  admixture  with  the  bile.  It  is 
propelled  onward  by  the  peristaltic  movement,  the  result  of  the  contraction 
of  the  circular  and  longitudinal  muscle-fibers. 

During  the  passage  of  the  digesting  food  through  the  intestinal  canal  the 
nutritive  products — the  amino-acids,  the  dextrose  and  levulose,  the  fatty 
emulsions,  the  fatty  acids  and  their  soaps — are  absorbed  into  the  blood,  while 
the  undigested  residue  is  carried  onward  by  the  peristaltic  movements  through 
the  ileo-cecal  valve  into  the  large  intestine. 

Intestinal  Fermentation. — Owing  to  the  favorable  conditions  for  fer- 
mentative and  putrefactive  processes — e.  g.,  heat,  moisture,  oxygen,  micro- 
organisms— the  food,  when  consumed  in  excessive  quantity  or  when  acted 
upon  by  defective  secretions,  undergoes  a  series  of  decomposition  changes 
which  are  attended  by  the  production  of  gases  and  various  chemic  compounds. 
Grape-sugar  and  maltose  are  partially  split  into  lactic  acid,  this  into  butyric 
acid,  carbon  dioxid,  and  hydrogen.  Fats  are  reduced  to  glycerol  and  fatty 
acids;  the  glycerol,  according  to  the  organisms  present,  yields  succinic  and 
other  fatty  acids,  carbon  dioxid,  and  hydrogen. 

The  proteins,  under  the  prolonged  action  of  the  pancreatic  juice,  are  decom- 
posed, and  yield  leucin  and  tyrosin;  the  former  is  split  into  valerianic  acid, 
ammonia,  and  carbon  dioxid;  the  latter  is  split  into  indol,  which  is  the  ante- 
cedent of  indican  in  the  urine.  Skatol  is  another  proteid  derivative  constantly 
present  in  the  fecal  substance. 


102  HUMAN  PHYSIOLOGY. 

Intestinal  Movements. — During  intestinal  digestion  the  walls  of  the 
intestine  exhibit  two  kinds  of  movement,  viz.,  a  rhythmic  segmentation  and  a 
peristalsis.  By  the  former  the  food  is  divided  into  segments  and  by  the 
latter,  it  is  carried  down  the  intestine.  Shortly  after  the  entrance  of  the  food 
into  the  intestine,  segmentation  begins  by  a  contraction  of  bands  of  circular 
muscle  fibers.  So  soon  as  a  mass  of  food  is  divided  into  segments  each  seg- 
ment is  in  turn  again  divided  by  similar  contractions.  The  lower  half  of  each 
segment  then  unites  with  the  upper  half  of  the  segment  below  to  commingle  with 
it  and  to  expose  new  surfaces  of  the  food  mass  to  contact  with  the  intestinal 
juices  and  to  the  mucous  membrane.  A  continual  repetition  of  this  process 
results  in  a  thorough  mixing  of  the  food  with  the  digestive  juices.  Subsequent 
peristaltic  waves  slowly  carry  the  food  down  the  intestine. 

The  large  intestine  extends  from  the  ileo-cecal  valve  to  the  anus,  and  is 
about  five  feet  in  length.  Like  the  stomach  it  consists  of  three  coats:  the 
serous,  the  muscular,  and  mucous.  The  mucous  membrane  contains  a 
number  of  mucous  glands,  the  secretion  from  which  lubricates  the  surface 
of  the  canal.  The  ascending  portion  of  the  large  intestine  possesses  the 
power  of  absorption,  and  hence  its  contents  become  less  liquid  and  more 
consistent.  As  the  residue  passes  toward  the  sigmoid  flexure  it  acquires 
the  characteristics  of  fecal  matter.  This  residue  consists  of  the  undigested 
portions  of  the  food,  decomposition  products,  mucus,  and  inorganic  salts. 

Defecation  is  the  voluntary  act  of  extruding  the  feces  from  the  rectum, 
and  is  accomplished  by  a  relaxation  of  the  sphincter  ani  muscle  and  by  the 
contraction  of  the  muscular  walls  of  the  rectum,  aided  by  the  contraction  of 
the  abdominal  muscles. 


ABSORPTION. 

The  term  absorption  is  applied  to  the  passage  or  transference  of  material 
into  the  blood  from  the  tissues,  from  the  serous  cavities,  and  from  the  mucous 
surfaces  of  the  body.  The  most  important  of  these  surfaces,  especially  in  its 
relation  to  the  formation  of  blood,  is  the  mucous  surface  of  the  alimentary 
canal;  for  it  is  from  this  organ  that  new  materials  are  derived  which  maintain 
the  quality  and  quantity  of  the  blood.  The  absorption  of  materials  from  the 
interstices  of  the  tissues  is  to  be  regarded  rather  as  a  return  to  the  blood  of 
liquid  nutritive  material  which  has  escaped  from  the  blood-vessels  for  nutri- 
tive purposes,  and  which,  if  not  returned,  would  lead  to  an  accumulation 
of  such  fluid  and  the  development  of  dropsical  conditions. 

The  anatomic  mechanisms  involved   in   the   absorptive   processes  are, 


ABSORPTION.  103 

primarily,  the  lymph-spaces,  the  lymph-capillaries,  and  the  blood-capillaries; 
secondarily,  the  lymphatic  vessels  and  larger  blood-vessels. 

Lymph-spaces,  Lymph-capillaries,  Blood-capillaries. — Everywhere 
throughout  the  body,  in  the  intervals  between  connective-tissue  bundles  and 
in  the  interstices  of  the  several  structures  of  which  an  organ  is  composed,  are 
found  spaces  of  irregular  shape  and  size,  determined  largely  by  the  nature  of 
the  organ  in  which  they  are  found,  which  have  been  termed  lymph-space:  or 
lacunce,  from  the  fact  that  during  the  living  condition  they  are  continually 
receiving  the  lymph  which  has  escaped  from  the  blood-vessels  throughout 
the  body.  In  addition  to  the  connective-tissue  lymph-spaces,  various  ob- 
servers have  described  special  lymph-spaces  in  the  testicle,  kidney,  liver,  thy- 
mus gland,  and  spleen;  in  all  secreting  glands  between  the  basement  membrane 
and  blood-vessels;  around  blood-vessels  ^perivascular  spaces),  and  around 
nerves.  The  serous  cavities  of  the  body — peritoneal,  pleural,  pericardial, 
etc. — may  also  be  regarded  as  lymph-spaces,  which  are  in  direct  communi- 
cation by  open  mouths  or  stomata  with  the  lymph  capillaries.  This  method 
of  communication  is  not  only  true  of  serous  membranes,  but  to  some  extent 
also  of  mucous  membranes.  The  cylindric  sheaths  and  endothelial  cells 
surrounding  the  brain,  spinal  cord,  and  nerves  can  also  be  looked  upon  as 
lymph-spaces  in  connection  with  lymph-capillaries. 

The  lymph-capillaries ,  in  which  the  lymph-vessels  proper  take  their  origin, 
are  arranged  in  the  form  of  plexuses  of  quite  irregular  shape.  In  most 
situations  they  are  intimately  interwoven  with  the  blood-vessels,  from  which, 
however,  they  can  be  readily  distinguished  by  their  larger  caliber  and  irreg- 
ular expansions.  The  wall  of  the  lymph-capillary  is  formed  by  a  single 
layer  of  epithelioid  cells,  with  sinuous  outlines,  and  which  accurately  dove- 
tail with  one  another.  In  no  instance  are  valves  found.  In  the  villus  of  the 
small  intestine  the  beginning  of  the  lymphatic  is  to  be  regarded  as  a  lymph- 
capillary,  generally  club-shaped,  which  at  the  base  of  the  villus  enters  a 
true  lymphatic;  at  this  point  a  valve  is  situated,  which  prevents  regurgitation. 
The  lymph  capillaries  anastomose  freely  with  one  another,  and  communic- 
cate  on  the  one  hand  with  the  lymph-spaces,  and  on  the  other  with  the  lym- 
phatic vessels  proper. 

As  the  shape,  size,  etc.,  of  both  lymph-spaces  and  capillaries  are  deter- 
mined largely  by  the  nature  of  the  tissues  in  which  they  are  contained,  it  is 
not  always  possible  to  separate  the  one  from  the  other.  Their  function, 
however,  may  be  regarded  as  similar — viz.,  the  collection  of  the  lymph  which 
has  escaped  from  the  blood-vessels,  and  its  transmission  onward  into  the 
regular  lymphatic  vessels. 

The  blood-capillaries  not  only  permit  the  escape  of  the  liquid  nutritive 


104  HUMAN  PHYSIOLOGY. 

portions  of  the  blood  through  their  delicate  walls,  but  are  also  engaged  in  the 
reabsorption  of  this  transudate,  as  well  as  in  the  absorption  of  new  materials 
from  the  alimentary  canal.  The  extensive  capillary  network  which  is  formed 
by  the  ultimate  subdivision  of  the  arterioles  in  the  submucous  tissue  and 
villi  of  the  small  intestine  forms  an  anatomic  arrangement  well  adapted  for 
absorption.  It  is  now  well  known  that  in  the  absorption  of  the  products  of 
digestion  the  blood-capillaries  are  more  active  than  the  lymph-capillaries. 

Lymph-vessels. — These  constitute  a  system  of  minute,  delicate  trans- 
parent vessels,  found  in  nearly  all  the  organs  and  tissues  of  the  body.  Having 
their  origin  at  the  periphery  in  the  lymph- capillaries  and  spaces,  they  rapidly 
converge  toward  the  trunk  of  the  body  and  empty  into  the  thoracic  duct.  In 
their  course  they  pass  through  numerous  small  ovoid  bodies,  the  lymphatic 
glands. 

The  lymph-vessels  of  the  small  intestines — the  lacteals — arise  within  the 
villous  processes  which  project  from  the  inner  surface  of  the  intestine  through- 
out its  entire  extent.  The  wall  of  the  villus  is  formed  by  an  elevation  of  the 
basement  membrane,  and  is  covered  by  a  layer  of  columnar  epithelial  cells. 
The  basis  of  the  villus  consists  of  adenoid  tissue,  a  fine  plexus  of  blood-vessels, 
unstriped  muscle-fibers,  and  the  lacteal  vessel.  The  adenoid  tissue  consists 
of  a  number  of  intercommunicating  spaces,  containing  leukocytes.  The 
lacteal  vessel  possesses  a  thin  but  distinct  wall  composed  of  endothelial 
plates,  with  here  and  there  openings  which  bring  the  interior  of  the  villus 
into  communication  with  the  spaces  of  the  adenoid  tissue. 

The  structure  of  the  larger  vessels  resembles  that  of  the  veins,  consisting 
of  three  coats: 

i.  External,  composed  of  fibrous  tissue  and  muscle  fibers,  arranged  longi- 
tudinally. 

2.  Middle,  consisting  of  white  fibers  and  yellow  elastic  tissue,  nonstriated 
muscle-fibers,  arranged  transversely. 

3.  Internal,  composed  of  an  elastic  membrane,  lined  by  endothelial  cells. 
Throughout  their  course  are  found  numerous  semilunar  valves,  opening 

toward  the  larger  vessels,  formed  by  a  folding  of  the  inner  coat  and  strength- 
ened by  connective  tissue. 

Lymph  Glands. — The  lymph  glands  consist  of  an  external  capsule  com- 
posed of  fibrous  tissue  which  contains  non-striped  muscle-fibers;  from  its 
inner  surface  septa  of  fibrous  tissue  pass  inward  and  subdivide  the  gland- 
substance  into  a  series  of  compartments,  which  communicate  with  one 
another.  The  blood-vessels  which  penetrate  the  gland  are  surrounded  by 
fine  threads,  forming  a  follicular  arrangement,  the  meshes  of  which  contain 
numerous  lymph-corpuscles.     Between  the  follicular  threads  and  the  wall 


ABSORPTION. 


105 


Fig.  15. — Diagram  Showing  the  Course  of  the  Main  Trunks  of  the  Absorbent 
System. — (Yeo's  "  Text-book  of  Physiology.") 
The  lymphatics  of  lower  extremities  (D)  meet  the  lacteals  of  intestines  (LAC)  at 
the  receptaculum  chyli  (RC),  where  the  thoracic  duct  begins.  The  superficial 
vessels  are  shown  in  the  diagram  on  the  right  arm  and  leg  (S),  and  the  deeper  ones 
on  the  arm  to  the  left  (D).  The  glands  are  here  and  there  shown  in  groups.  The 
small  right  duct  opens  into  the  veins  on  the  right  side.  The  thoracic  duct  opens  into 
the  union  of  the  great  veins  of  the  left  side  of  the  neck  (T). 


106  HUMAN  PHYSIOLOGY. 

of  the  gland  lies  a  lymph-channel  traversed  by  a  reticulum  of  adenoid  tissue. 
The  lymph-vessels,  after  penetrating  this  capsule,  pour  their  lymph  into  this 
channel,  through  which  it  passes;  it  is  then  collected  by  the  efferent  vessels 
and  transmitted  onward.  The  lymph-corpuscles  which  are  washed  out  of 
of  the  gland  into  the  lymph-stream  are  formed,  most  probably,  by  division 
of  preexisting  cells. 

The  thoracic  duct  is  the  general  trunk  of  the  lymphatic  system;  into  it  the 
vessels  of  the  lower  extremities,  of  the  abdominal  organs,  of  the  left  side  of 
the  head,  and  of  the  left  arm  empty  their  contents.  It  is  about  twenty  inches 
in  length,  arises  in  the  abdomen,  opposite  the  third  lumbar  vertebra,  by  a 
dilatation  (the  receptaculum  chyli),  ascends  along  the  vertebral  column  to  the 
seventh  cervical  vertebra,  and  terminates  in  the  venous  system  at  the  junction 
of  the  internal  jugular  and  subclavian  veins  on  the  left  side.  The  lymphatics 
of  the  right  side  of  the  head,  of  the  right  arm,  and  of  the  right  side  of  the 
thorax  terminate  in  the  right  thoracic  duct,  about  one  inch  in  length,  which 
joins  the  venous  system  at  the  junction  of  the  internal  jugular  and  subclavian 
on  the  right  side. 

The  general  arrangement  of  the  lymph  vessels  is  shown  in  figure  15. 

The  blood-vessels  which  are  concerned  in  the  conduction  of  fresh  nutri- 
tive material  from  the  alimentary  canal  have  their  origin  in  the  elaborate 
capillary  network  in  the  mucous  membrane.  The  small  veins  which  emerge 
from  the  network  gradually  unite,  forming  larger  and  larger  trunks,  which  are 
known  as  the  gastric,  superior,  and  inferior  mesenteric  veins.  These  finally 
unite  to  form  the  portal  vein,  a  short  trunk  about  three  inches  in  length. 
The  portal  vein  enters  the  liver  at  the  transverse  fissure,  after  which  it  forms 
a  fine  capillary  plexus  ramifying  throughout  the  substance  of  the  liver;  from 
this  plexus  the  hepatic  veins  take  their  origin,  and  finally  empty  the  blood 
into  the  vena  cava  inferior.     (See  Fig.  16.) 

Absorption  of  Food. — Physiological  experiments  have  demonstrated  that 
the  agents  concerned  in  the  absorption  of  new  materials  from  the  alimentary 
canal  are: 

1.  The  blood-vessels  of  the  entire  canal,  but  more  particularly  those  uniting 
to  form  the  portal  vein. 

2.  The  lymph  vessels  coming  from  the  small  intestine,  which  converge  to 
empty  into  the  thoracic  duct. 

As  a  result  of  the  action  of  the  digestive  fluids  upon  the  different  classes 
of  food  principles — proteins,  sugars,  starches,  and  fats — there  are  formed 
amino-acids,  dextrose  and  levulose,  soap  and  glycerin,  which  differ  from 
the  former  in  being  highly  diffusible — a  condition  essential  to  their  absorption. 


ABSORPTION. 


107 


Their  absorption  is  accomplished  by  the  villous  processes  covering  the 
surface  of  the  intestinal  mucous  membrane. 

The  villi  are  small  filiform  or  conical  processes  projecting  from  the  surface 
of  the  mucous  membrane.  Each  villus  consists  of  a  basement  membrane 
supporting  columnar  epithelial  cells.  In  the  interior'of  the  villus  there  is 
frame  work  of  connective  tissue  supporting  arteries,  capillaries  and  veins 
and  a  single  club-shaped  lymph  capillary. 


Diagram  of  the  portal  vein  (pv)  arising  in  the  alimentary  tract  and  spleen  (s),  and 
carrying  the  blood  from  these  organs  to  the  liver. — (Yeo's  "  Text-book  of  Physiology   .) 


The  function  of  the  epithelium  is  the  absorption  of  the  products  of 
digestion. 

The  water  inorganic  salts  and  the  sugars  after  their  absorption  pass  onward 
into  the  interior  of  the  villi;  thence  through  the  capillary  wall  into  the  blood 
by  which  they  are  carried  to  the  liver. 

The  amino-acids  after  absorption  are  synthesized  in  part  to  a  form  of 
protein,  similar,  if  not  identical,  with  that  present  in  the  blood  plasma. 


108  HUMAN  PHYSIOLOGY. 

It  is  then  passed  onward  into  the  interior  of  the  villus  to  enter  the  blood 

stream. 

The  soap  and  glycerin  after  absorption  are  synthesized  to  fat  which  is 

deposited  in  the  epithelial  cells  in  the  form  of  small  drops,  after  which  it 

too  passes  to  the  interior  of  the  villus  to  enter  the  lymph  capillary. 

The  products  of  digestion  find  their  way  into  the  general  circulation  by 

two  routes: 

i.  The  water,  protein,  dextrose,  and  soluble  salts,  after  passing  into  the  lymph- 
spaces  of  the  villi,  pass  through  the  wall  of  the  capillary  blood-vessel; 
entering  the  blood,  they  are  carried  to  the  liver  by  the  vessels  uniting  to 
form  the  portal  vein;  emerging  from  -the  liver,  they  are  emptied  into  the 
inferior  vena  cava  by  the  hepatic  vein. 

2.  The  fat  enters  the  lymph-capillary  in  the  interior  of  the  villus;  by  the 
contraction  of  the  layer  of  muscle-fibers  surrounding  it  its  contents  are 
forced  onward  into  the  lymph-vessels  or  lacteals,  thence  into  the  thoracic 
duct,  and  finally  into  the  blood  stream  at  the  junction  of  the  internal 
jugular  and  subclavian  veins  on  the  left  side. 
Properties  and  Composition  of  Lymph. — Lymph,   as  found  in  the 

lymph-vessels  of  animals,  is  a  clear,  colorless,  or  opalescent  fluid,  having 

an    alkaline    reaction,     a    saline    taste,    and    a   specific   gravity   of   about 

1040.     It  holds   in  suspension  a  number  of  corpuscles  resembling  in  their 

COMPOSITION   OF  LYMPH. 

.  Water 95 .536 

Proteins  (serum-albumin,  fibrin-globulin) 1 .320 

Extractives  (urea,  sugar,  cholesterin) 1 .  559 

Fatty  matters : a  trace 

Salts o .  585 


100.000 


general  appearance  the  white  corpuscles  of  the  blood.  Their  number  has 
been  estimated  at  8,200  per  cubic  millimeter,  though  the  number  varies  in 
different  portions  of  the  lymphatic  system.  As  the  lymph  flows  through 
the  lymphatic  gland  it  receives  a  large  addition  of  corpuscles.  Lymph- 
corpuscles  are  granular  in  structure,  and  measure  2^00  °f  an  mcn  in 
diameter.  When  withdrawn  from  the  vessels,  lymph  undergoes  a  spon- 
taneous coagulation  similar  to  that  of  blood,  after  which  it  separates  in 
serum  and  clot. 

Origin  and  Functions  of  Lymph. — Though  the  blood  is  the  common 
reservoir  of  all  nutritive  materials,  they  are  not  available  for  nutritive  purposes 


ABSORPTION.  109 

as  long  as  they  are  confined  within  the  blood-vessels.  But  owing  to  the  char- 
acter of  the  wall  of  the  capillary  blood-vessels,  some  of  the  constituents  of 
the  blood-plasma  pass  through  it  and  are  received  by  the  tissue-spaces  in 
which  they  come  into  contact  with  the  tissue-cells.  To  the  sum  total  of 
these  materials  the  term  lymph  is  given.  Its  function  becomes  apparent 
from  its  origin  and  composition,  its  situation  and  relation  to  the  tissues.  It 
is  to  furnish  the  tissue-cells  with  those  nutritive  materials  which  are  necessary 
for  their  growth,  repair  and  functional  activity.  It  also  receives  all  waste 
products  that  arise  from  their  activity  prior  to  their  removal  by  the  blood- 
and  lymph-vessels. 

Absorption  of  Lymph. — From  the  fact  that  lymph  is  being  discharged 
more  or  less  continuously  from  the  thoracic  duct,  it  is  evident  that  lymph 
is  being  absorbed  from  the  intercellular  spaces;  from  which  it  may  be  inferred 
that  more  lymph  is  passing  from  the  blood  into  the  tissue-spaces  than  is 
necessary  for  the  immediate  needs  of  the  tissues.  To  prevent  an  accumula- 
tion and  an  interference  through  pressure  with  the  activities  of  the  tissues, 
the  excess  is  absorbed  by  the  lymph-vessels  and  returned  to  the  blood  stream 
by  way  of  the  thoracic  duct.  It  is  likely  that  some  of  the  constituents  are 
absorbed  by  the  blood-vessels. 

Chyle. — Chyle  is  the  fluid  found  in  the  lymph  vessels,  coming  from  the 
small  intestine  after  the  digestion  of  a  meal  containing  fat.  In  the  intervals 
of  digestion  the  fluid  of  these  lymphatics  is  identical  in  all  respect  with  the 
lymph  found  in  all  other  regions  of  the  body.  As  soon  at  the  granular 
fat  passes  into  the  lymph  vessels  and  mingles  with  the  lymph  it  becomes 
milky  white  in  color,  and  the  vessels  which  previously  were  invisible  become 
visible,  and  resemble  white  threads  running  between  the  layers  of  the  mesen- 
tery. Chyle  has  a  composition  similar  to  that  of  lymph,  but  it  contains, 
in  addition,  numerous  fatty  granules,  each  surrounded  by  an  albuminous 
envelope.  When  examined  microscopically,  the  chyle  presents  a  fine 
molecular  basis,  made  up  of  the  finely  divided  granules  of  fat. 

COMPOSITION   OF  CHYLE. 

Water 902 .37 

Albumin 35 .16 

Fibrinogen 3-7° 

Extractives 15 .  65 

Fatty  matters 36.01 

Salts 7. 11 

1,000.00 


1 10  HUMAN  PHYSIOLOGY. 

Forces  Aiding  the  Movement  of  Lymph  and  Chyle. — The  lymph  and 
chyle  are  continually  moving  in  a  progressive  manner  from  the  periphery  or 
beginning  of  the  lymphatic  system  to  the  final  termination  of  the  thoracic 
duct.  The  force  which  primarily  determines  the  movement  of  the  lymph 
has  its  origin  in  the  beginnings  of  the  lymph-vessels,  and  depends  upon  the 
difference  in  pressure  here  and  the  pressure  in  the  thoracic  duct.  The  greater 
the  quantity  of  fluid  poured  into  the  lymph-spaces,  the  greater  will  be  the 
pressure  and,  consequently,  the  movement.  The  first  movement  of  chyle 
is  the  result  of  a  contraction  of  the  muscle-fibers  within  the  walls  of  the  villus. 
At  the  time  of  contraction  the  lymph  capillary  is  compressed  and  shortened, 
and  its  contents  are  forced  onward  into  the  true  lymphatic.  When  the 
muscle-fibers  relax,  regurgitation  is  prevented  by  the  closure  of  the  valve  in 
the  lymphatic  at  the  base  of  the  villus. 

As  the  walls  of  the  lymph  vessels  contain  muscle-fibers,  when  they  become 
distended  these  fibers  contract  and  assist  materially  in  the  onward  move- 
ment of  the  fluid. 

The  contraction  of  the  general  muscular  masses  in  all  parts  of  the  body, 
by  exerting  an  intermittent  pressure  upon  the  lymphatics,  also  hastens  the 
current  onward;  regurgitation  is  prevented  by  the  closure  of  valves  which 
everywhere  line  the  interior  of  the  vessels. 

The  respiratory  movements  aid  the  general  flow  of  both  lymph  and  chyle 
from  the  thoracic  duct  into  the  venous  blood.  During  the  time  of  an  in- 
spiratory movement  the  pressure  within  the  thorax,  but  outside  the  lungs, 
undergoes  a  diminution  in  proportion  to  the  extent  of  the  movement;  as  a 
result,  the  fluid  in  the  thoracic  duct  outside  of  the  thorax,  being  under  a 
higher  pressure,  flows  more  rapidly  into  the  venous  system.  At  the  time  of 
an  expiration,  the  pressure  rises  and  the  flow  is  temporarily  impeded,  only  to 
begin  again  at  the  next  inspiration. 


BLOOD. 

The  blood  is  a  nutritive  fluid  containing  all  the  elements  necessary  for  the 
repair  of  the  tissues;  it  also  contains  principles  of  waste  absorbed  from  the 
tissues,  which  are  conveyed  to  the  various  excretory  organs  and  by  them 
eliminated  from  the  body. 

The  total  amount  of  blood  in  the  body  is  estimated  to  be  about  one  nineteenth 
of  the  body-weight;  from  six  to  eight  pounds  in  an  individual  of  average 
physical  development.  The  quantity  varies  during  the  twenty-four  hours, 
the  maximum  being  reached  in  the  afternoon,  the  minimum  in  the  early 
morning  hours. 


BLOOD.  Ill 

Blood  is  an  opaque,  red  fluid,  having  an  alkaline  reaction,  a  saline  taste, 
and  a  specific  gravity  of  1055. 

The  opacity  is  due  to  the  refraction  of  the  rays  of  light  by  the  elements  of 
which  the  blood  is  composed.  The  color  varies  in  hue,  from  a  scarlet  red 
in  the  arteries  to  a  bluish  red  in  the  veins,  due  to  the  presence  of  a  coloring - 
matter — hemoglobin — in  different  degrees  of  oxidation. 

The  alkalinity  is  constant,  and  depends  upon  the  presence  of  the  alkaline 
sodium  phosphate,  Na2HP04. 

The  saline  taste  is  due  to  the  amount  of  sodium  chlorid  present. 

The  specific  gravity,  within  the  limits  of  health,  ranges  from  1.045  to  I-°75> 
though  the  average  is  about  1.056. 

The  odor  of  the  blood  is  characteristic,  and  varies  with  the  animal  from 
which  is  it  drawn;  it  is  due  to  the  presence  of  caproic  acid. 

The  temperature  of  the  blood  ranges  from  980  F.  at  the  surface  to  1070  F. 
in  the  hepatic  vein;  it  loses  heat  by  radiation  and  evaporation  as  it  approaches 
the  extremities  and  as  it  passes  through  the  lungs. 

Blood  Consists  of  Two  Portions: 

1.  The  liquor  sanguinis  or  plasma,  a  transparent,  colorless  fluid,  in  which 
are  floating — 

2.  Red  and  white  corpuscles,  these  constituting  by  weight  less  than  one  half 
(40  per  cent.)  of  the  entire  amount  of  blood. 

COMPOSITION   OF   PLASMA. 

Water 902 .  00 

Albumin 53  .  00 

Paraglobulin 22 .  00 

Fibrinogen 3  .  00 

Fatty  matters 2.50 

Crystallizable  nitrogenous  matters 4.00 

Other  organic  matter 5  .00 

Mineral  salts 8.50 


1,000.00 


Water  acts  as  a  solvent  for  the  inorganic  matters  and  holds  in  suspension 
the  corpuscular  elements. 

Albumin  is  the  nutritive  principle  of  the  blood;  it  is  absorbed  by  the  tissues 
to  repair  their  waste  and  is  transformed  into  the  organic  basis  characteristic 
of  each  structure. 


112  HUMAN  PHYSIOLOGY. 

Paraglobulin  or  fibrinoplastin  is  a  soft,  amorphous  substance  precipitated 
by  sodium  chloride  in  excess,  or  by  passing  a  stream  of  carbonic  acid  through 
dilute  serum. 

Fibrinogen  also  can  be  obtained  by  strongly  diluting  the  serum  and  passing 
carbonic  acid  through  it  for  a  long  time,  when  it  is  precipitated  as  a  viscous 
deposit. 

Fatty  matter  exists  in  the  blood  to  the  extent  of  about  o .  25  per  cent.  Just 
after  a  meal  rich  in  fat,  this  amount  may  be  considerably  increased.  Within 
a  few  hours  it  disappears,  though  its  ultimate  fate  is  unknown. 

Sugar  is  represented  by  dextrose.  The  amount  present  varies  from  0.1 
to  0.3  per  cent.  It  is  derived  directly  from  the  glycogen  of  the  liver.  Should 
the  normal  percentage  be  increased,  the  sugar  is  eliminated  by  the  kidneys. 

The  inorganic  constituents  are  chiefly  sodium  and  potassium  chlorids, 
sulphates  and  phosphates  together  with  calcium  and  magnesium  phosphates. 
The  sodium  chlorid  is  the  most  abundant,  amounting  to  about  5.5  parts  per 
thousand.  The  alkaline  salts  impart  the  alkaline  reaction  and  promote  the 
absorption  from  the  tissues  of  the  carbon  dioxid. 

Excrementitious  matters  are  represented  by  carbonic  acid,  urea,  creatin, 
creatinin,  urates,  oxalates,  etc. ;  they  are  absorbed  from  the  tissues  by  the 
blood  and  conveyed  to  the  excretory  organs,  lungs,  kidneys,  etc. 

Gases. — Oxygen,  nitrogen,  and  carbonic  acid  exist  in  varying  proportions. 

BLOOD-CORPUSCLES. 

The  corpuscular  elements  of  the  blood  occur  under  two  distinct  forms, 
which,  from  their  color,  are  known  as  the  red  and  white  corpuscles. 

The  red  corpuscles  as  they  float  in  a  thin  layer  of  the  liquor  sanguinis  are 
of  a  pale  straw-color;  it  is  only  when  aggregated  in  masses  that  they  assume 
the  bright  red  color.  In  form  they  are  circular  and  biconcave ;  they  have  an 
average  diameter  of  ^zoo  °f  an  inch. 

In  mammals,  birds,  reptiles,  amphibia,  and  fish  the  corpuscles  vary  in 
size  and  number,  gradually  becoming  larger  and  less  numerous  as  the  scale 
of  animal  lif e  is  descended,  e.  g. : 

TABLE  SHOWING  COMPARATIVE  DIAMETER  OF  RED  CORPUSCLES. 


Mammals. 

Birds. 

Reptiles 

Amphibia. 

Fisl 

Man, 

33rra 

Eagle, 

IS1!  2 

Turtle, 

T23T 

Frog, 

TI0~5 

Perch, 

Chimpanzee, 

34l2 

Owl, 

1783 

Tortoise, 

T2V2 

Toad, 

TS43 

Carp, 

Orang, 

33V3 

Sparrow, 

1 
2140 

Lizard, 

1 
1555 

Proteus, 

4  00 

Pike, 

Dog, 

35*42 

Swallow, 

2133 

Viper, 

1 
1274 

Siren, 

1 

4  20 

Eel, 

Cat, 

1 

4404 

Pigeon, 

T573 

Amphiuma, 

3  S3 

Hog, 

tJtsG 

Turkey, 

2045 

Horse, 

4600 

Goose, 

T 

1566 

Ox, 

5257 

Swan, 

1506 

BLOOD.  113 

In  man  and  the  mammals  the  red  corpuscles  present  neither  a  nucleus  nor 
a  cell  wall,  and  are  universally  of  a  small  size.  They  can  be  readily  dis- 
tinguished from  the  corpuscles  of  birds,  reptiles,  and  fish,  in  which  animals 
they  are  larger,  oval  in  shape,  and  possess  a  well-defined  nucleus. 

The  red  corpuscles  are  exceedingly  numerous,  amounting  to  about  5,000,000 
in  a  cubic  millimeter  of  blood.  In  structure  they  consist  of  a  firm,  elastic, 
colorless  framework — the  stroma — in  the  meshes  of  which  is  entangled  the 
coloring-matter — the  hemoglobin. 


CHEMIC   COMPOSITION    OF   RED    CORPUSCLES. 

Water 68.80 

Hemoglobin 30 .  00 

Fatty  matter o .  23 

Extractives 0.16 

Mineral  salts 0.81 


100.00 


Hemoglobin,  the  coloring-matter  of  the  corpuscles,  is  an  albuminous 
compound,  composed  of  C,0,H,N,S,  and  iron.  It  may  exist  in  either 
an  amorphous  or  a  crystalline  form.  When  deprived  of  all  its  oxygen, 
except  the  quantity  entering  into  its  intimate  composition,  the  hemoglobin 
becomes  purplish  in  color,  and  is  known  as  reduced  hemoglobin.  When  ex- 
posed to  the  action  of  oxygen,  it  again  absorbs  a  definite  amount  and  becomes 
scarlet  in  color,  and  is  known  as  oxyhemoglobin.  The  amount  of  oxygen 
absorbed  is  1.34  c.c.  for  each  gram  of  hemoglobin. 

It  is  this  substance  which  gives  the  color  to  the  venous  and  arterial  blood . 
As  the  venous  blood  passes  through  the  capillaries  of  the  lungs  the  reduced 
hemoglobin  absorbs  the  oxygen  from  the  pulmonary  air  and  becomes  oxy- 
hemoglobin, scarlet  in  color;  the  blood  becomes  arterial.  When  the  arterial 
blood  passes  into  the  systemic  capillaries,  the  oxygen  is  absorbed  by  the  tis- 
sues; the  hemoglobin  becomes  reduced,  purple  in  color,  and  the  blood 
becomes  venous.  A  dilute  solution  of  oxyhemoglobin  gives  two  absorption 
bands  between  the  lines  D  and  E  of  the  solar  spectrum.  Reduced  hemo- 
globin gives  but  one  absorption  band,  occupying  the  space  existing  between 
the  two  bands  of  the  oxyhemoglobin  spectrum. 

^he  function  of  the  red  corpuscle  is,  therefore,  to  absorb  oxygen  and  carry 
it  to  the  tissues;  the  smaller  the  corpuscles  and  the  greater  the  number,  the 
8 


114  HUMAN  PHYSIOLOGY. 

greater  is  the  quantity  of  oxygen  absorbed,  and,  consequently,  all  the  vital 
functions  of  the  body  become  more  active. 

The  white  corpuscles  are  far  less  numerous  than  the  red,  the  proportion 
being,  on  an  average,  about  i  white  to  from  350  to  400  red;  they  are  globular 
in  shape,  and  are  2  sV  0  °f  an  mcn  m  diameter,  and  consist  of  a  soft,  granu- 
lar, colorless  substance,  containing  several  nuclei. 

The  number  per  cubic  millimeter  varies  from  7,000  to  10,000. 

Five  distinct  varieties  of  white  corpuscles  are  now  recognized,  viz.:  1. 
small  lymphocytes;  2.  large  lymphocytes;  3.  Polymorphonuclear  leuko- 
cytes; 4.  Eosinophiles;  5.  Basophiles. 

The  white  corpuscles  possess  the  power  of  spontaneous  movement,  alter- 
nately contracting  and  expanding,  throwing  out  processes  of  their  substance 
and  quickly  withdrawing  them,  thus  changing  their  shape  from  moment  to 
moment.  These  movements  resemble  those  of  the  ameba,  and  for  this 
reason  are  termed  ameboid.  The  white  corpuscles  also  possess  the  capability 
of  passing  through  the  walls  of  the  capillaries  into  the  surrounding  tissue 
spaces;  to  this  process  the  term  diapedesis  is  given. 

The  white  corpuscles  are  identical  with  the  leukocytes,  and  are  found  in 
milk,  lymph,  chyle,  and  other  fluids. 

The  function  of  the  leucocytes  is  imperfectly  known.  It  has  been  sug- 
gested that  they  are  engaged  in  the  removal  of  dead  tissue,  of  attacking  and 
destroying  bacteria.     For  this  reason  they  have  been  termed  phagocytes. 

Origin  of  Corpuscles. — The  red  corpuscles  are  developed  out  of  nucle- 
ated cells,  the  erythroblasts  found  in  the  red  marrow  of  the  long  bones. 
The  latter  increase  by  karyokinesis  and  increase  in  their  hemoglobin  content. 
The  nucleus  is  finally  extruded,  after  which  the  cell  assumes  the  form  char- 
acteristic of  the  red  corpuscle. 

The  white  corpuscles  arise  from  two  different  sources.  The  lymphocytes 
take  their  origin  from  the  lymph-adenoid  tissues,  e.  g.,  lymph  glands,  solitary 
glands,  etc.  The  leucocytes  are  derived  from  the  myelocytes  found  in  the 
marrow" of  long  bones. 

COAGULATION  OF  THE  BLOOD. 

When  blood  is  withdrawn  from  the  body  and  allowed  to  remain  at  rest, 
it  becomes  somewhat  thick  and  viscid  in  from  three  to  five  minutes;  this 
viscidity  gradually  increases  until  the  entire  volume  of  blood  assumes  a  jelly- 
like consistence,  which  process  occupies  from  five  to  fifteen  minutes. 

As  soon  as  coagulation  is  completed,  a  second  process  begins,  which  con- 
sists in  the  contraction  of  the  coagulum  and  the  oozing  of  a  clear,  straw- 


BLOOD. 


115 


colored  liquid — the  serum — which  gradually  increases  in  quantity  as  the 
clot  diminishes  in  size,  by  contraction,  until  the  separation  is  completed, 
which  occupies  from  twelve  to  twenty-four  hours. 

The  changes  in  the  blood  are  as  follows: 

Before  coagulation. 


Living  blood.  < 


Liq.  Sanguinis, 
or 


t  consisting  of 


Plasma, 

Corpuscles,  red  and  white 
After  coagulation. 

Crassamentum.  ) 

Clot  or  coagulum,        ( 


Dead   blood. 


^  containing 


Serum, 


containing 


f  Water 
J  Albumin. 
Fibrinogen. 

Salts. 


J  Fibrin. 

\  Corpuscles. 

[  Water. 

Albumin. 

Salts. 


The  serum,  therefore,  differs  from  the  liquor  sanguinis  in  not  containing 
fibrin. 

In  from  twelve  to  twenty-four  hours  the  upper  surface  of  the  clot  presents 
a  grayish  appearance — the  huffy  coat — owing  to  the  rapid  sinking  of  the  red 
corpuscles  beneath  the  surface,  permitting  the  fibrin  to  coagulate  without 
them;  this  substance  then  assumes  a  grayish-yellow  tint.  Inasmuch  as  the 
white  corpuscles  possess  a  lighter  specific  gravity  than  the  red,  they  do  not 
sink  so  rapidly,  and,  becoming  entangled  in  the  fibrin,  assist  in  forming  the 
buffy  coat.  Continued  contraction  gives  a  cupped  appearance  to  the  surface 
of  the  clot. 

Inflammatory  states  of  the  blood  produce  a  marked  increase  in  the  buffed 
and  cupped  condition,  on  account  of  the  aggregation  of  the  corpuscles  and 
their  tendency  to  rapid  sinking. 

The  Cause  of  Coagulation. — Coagulation  is  due  to  the  appearance  of 
fibrin,  a  derivative  of  an  antecedent  substance  fibrinogen;  the  cause  of  the 
conversion  of  the  soluble  fibrinogen  into  the  insoluble  fibrin  is  the  presence 
and  activity  of  an  agent  termed  thrombin.  This  agent  is  believed  to  be 
a  derivative  of  an  antecedent  substance  prothrombin  or  thrombogen  a  sub- 
stance always  present  in  the  blood  and  is  a  product  of  the  decomposition  of 
leucocytes  and  the  blood  platelets.  With  thrombogen  there  is  associated  a 
calcium  salt  which  is  essential  for  coagulation.  If  it  is  removed  by  the 
addition  of  potassium  oxalate  coagulation  does  not  take  place.  These 
three  substances  prothrombin  or  thrombogen,  a  calcium  salt  and  fibrinogen 


Il6  HUMAN  PHYSIOLOGY. 

are  always  present  in  the  blood.  The  formation  of  thrombin  which  would 
cause  coagulation  is  prevented  by  the  presence  of  an  anti-thrombin.  As 
soon  as  blood  is  shed  or  tissues  are  injured  a  new  substance  thrombinoplastin 
is  developed  which  neutralizes  the  anti-thrombin.  This  having  been  accom- 
plished the  calcium  is  enabled  to  activate  the  prothrombin  with  the  produc- 
tion of  thrombin  and  hence  fibrin  (Howell.) 

Conditions  Influencing  Coagulation. — The  process  is  retarded  by  cold, 
retention  within  living  normal  vessels,  neutral  salts  in  excess,  the  injection 
of  commercial  peptone,  etc. 

It  is  accelerated  by  a  temperature  of  ioo°  F.,  contact  with  rough  surfaces, 
the  presence  of  foreign  bodies,  whipping,  etc. 

Blood  coagulates  in  the  body  after  the  arrest  of  the  circulation  in  the 
course  of  twelve  to  twenty-four  hours;  local  arrest  of  the  circulation,  from 
compression  or  a  ligature,  will  cause  coagulation,  thus  preventing  hemorrhages 
from  wounded  vessels. 

The  composition  of  the  blood  varies  in  different  portions  of  the  body. 
The  arterial  differs  from  the  venous,  in  being  more  coagulable;  in  containing 
more  oxygen  and  less  carbonic  acid;  in  having  a  bright  scarlet  color,  from  the 
union  of  oxygen  with  hemoglobin.  The  bluish  red  color  of  venous  blood 
results  from  the  deoxidation  of  the  coloring-matter. 

The  blood  of  the  portal  vein  differs  in  constitution,  according  to  different 
stages  of  the  digestive  process;  during  digestion  it  is  richer  in  water,  protein 
matter,  and  sugar;  occasionally  it  contains  fat;  corpuscles  are  diminished, 
and  there  is  an  absence  of  biliary  substances. 

The  blood  of  the  hepatic  vein  contains  a  larger  proportion  of  red  and  white 
corpuscles;  the  sugar  is  augmented,  while  protein  and  fat  are  diminished. 

CIRCULATION  OF  THE  BLOOD. 

The  circulatory  apparatus  by  which  the  blood  is  distributed  to  all 
portions  of  the  body  consists  of  a  central  organ — the  heart — with  which 
is  connected  a  system  of  closed  vessels  known  as  arteries,  capillaries,  and 
veins.  Within  this  system  the  blood  is  kept,  by  the  action  of  the  heart,  in 
continual  movement,  distributing  nutritive  matter  to  all  portions  of  the  body 
and  carrying  waste  matters  from  the  tissues  to  the  various  eliminating  organs. 

The  heart  is  a  hollow,  muscular  organ,  pyramidal  in  shape,  measuring 
about  5 \  inches  in  length  and  about  3I  in  breadth,  weighing  from  10 
to  12  ounces  in  the  male  and  from  8  to  10  in  the  female.  Situated  in  the 
thoracic  cavity,  between  the  lungs,  its  base  is  directed  upward,  backward, 
and  to  the  right;  its  apex  is  directed  downward  and  to  the  left. 


CIRCULATION    OF    THE    BLOOD.  117 

Pericardium. — The  heart  is  surrounded  by  a  closed  fibrous  membrane 
called  the  pericardium.  The  inner  surface  of  this  membrane  is  lined  by  a 
serous  membrane,  which  is  also  reflected  over  the  surface  of  the  heart; 
between  the  two  surfaces  of  the  serous  membrane  is  found  a  small  quantity 
of  fluid  (the  pericardial  fluid),  which  lubricates  the  surfaces  and  prevents 
friction  during  the  movements  of  the  heart.  The  interior  of  the  heart  is  also 
lined  by  a  serous  membrane,  called  the  endocardium. 

Cavities  of  the  Heart. — The  general  cavity  of  the  heart  is  sub-divided 
by  a  longitudinal  septum  into  a  right  and  left  half;  each  of  these  cavities  is 
in  turn  subdivided  by  a  transverse  septum  into  two  smaller  cavities,  which 
communicate  with  each  other  and  are  known  as  the  auricles  and  ventricles, 
the  orifice  between  the  auricle  and  ventricle  being  known  as  the  auriculo- 
ventricular  orifice.  The  heait,  therefore,  consists  of  four  cavities — a  right 
auricle  and  ventricle  and  a  left  auricle  and  ventricle. 

Into  the  right  auricle  the  two  terminal  trunks  of  the  venous  system — the 
superior  and  inferior  vena  cava — empty  the  venous  blood  which  has  been 
collected  from  all  parts  of  the  system;  from  the  right  ventricle  arises  the 
pulmonary  artery,  which,  passing  into  the  lungs,  distributes  the  blood  to  the 
walls  of  the  air-cells  of  the  lungs;  into  the  left  auricle  empty  four  pulmonary 
veins,  which  have  collected  the  blood  from  the  lung  capillaries;  from  the  left 
ventricle  springs  the  aorta,  the  general  trunk  of  the  arterial  system,  the 
branches  of  which  distribute  the  blood  to  the  entire  system. 

The  Valves  of  the  Heart. — The  valves  of  the  heart  are  formed  by  a 
reduplication  of  the  endocardium  strengthened  by  connective  tissue.  At 
the  auriculoventricular  openings  on  the  right  and  left  sides  of  the  heart, 
respectively,  are  found  the  tricuspid  and  mitral  valves.  The  tricuspid  valve 
consists  of  three,  the  mitral  of  two,  cusps  or  segments,  which  project  into  the 
interior  of  the  ventricle  when  it  does  not  contain  blood.  At  their  bases  the 
segments  are  united  so  as  to  form  an  annular  membrane  attached  to  the 
margin  of  the  orifice.  To  the  free  edges  of  the  valves  are  attached  numerous 
fine  threads — the  chorda  tendinece — which  are  the  tendons  of  the  small 
papillary  muscles  springing  from  the  walls  of  the  ventricles. 

The  Semilunar  Valves. — At  the  openings  of  the  pulmonary  artery  and 
the  aorta  are  found  three  cup-shaped  or  semilunar  valves,  the  free  edges  of 
which  are  directed  away  from  the  interior  of  the  heart.  The  anatomic 
arrangement  of  the  valves  is  such  that  upon  their  closure  regurgitation  of  the 
blood  is  prevented. 

The  Course  of  the  Blood  through  the  Heart. — Reference  to  figure  17  will 
make  it  clear  that  there  is  a  pathway  for  the  blood  between  the  venae  cavae 


n8 


HUMAN  PHYSIOLOGY. 


on  the  right  side  and  the  aorta  on  the 
left  side  by  way  of  the  right  side  of  the 
heart,  the  cardio-pulmonary  vessels  and 
the  left  side  of  the  heart. 

The  venous  blood  flowing  towards 
the  heart  is  emptied  by  the  superior 
and  inferior  venae  cavae  into  the  right 
auricle  from  which  it  passes  through 
the  auriculo ventricular  opening  into  the 
right  ventricle;  thence  into  and  through 
the  pulmonary  artery  and  its  branches 
to  the  pulmonary  capillaries  where  it  is 
arterialized,  i.e.,  yields  up  a  portion  of 
its  carbon  dioxid  and  takes  on  a  fresh 
supply  of  oxygen — and  is  changed  in 
color  from  bluish-red  to  scarlet-red. 
The  arterialized  blood  flowing  towards 
the  heart  is  emptied  by  the  pulmonary 
veins  into  the  left  auricle  from  which  it 
passes  through  the  auriculoventricular 
opening  into  the  left  ventricle;  thence 
into  the  aorta  and  its  branches  to  the 
systemic  capillaries  where  it  is  dear- 
terialized  by  an  opposite  exchange  of 
gases,  i.  e.,  yields  up  a  portion  of  its 
oxygen  to,  and  absorbs  carbon  dioxid 
from  the  tissues,  and  changes  in  color, 
from  scarlet-red  to  bluish-red.  The 
venous  blood  is  again  returned  to  the 
systemic  veins  to  the  venae  cavae. 

While  there  is  but  one  circulation, 
physiologists  frequently  divide  the  cir- 
culatory apparatus  into — 
i.  The  systemic  circulation,  which  in- 
cludes the  movement  of  the  blood  from 
the  left  side  of  the  heart  through  the 
aorta  and  its  branches,  through  the 
capillaries  and  veins,  to  the  right  side. 

Fig.  17. — Diagram  of  Circulation. 
1.  Heart.     2.  Lungs.     3.  Head  and  upper 
extremities.      4.  Spleen.      5.  Intestine.      6. 
Kidney.      7.  Lower  extremities.      8.  Liver. 
— (Dalton.) 


CIRCULATION   OF   THE   BLOOD.  119 

2.  The  pulmonary  circulation,  which  includes  the  course  of  the  blood  from 
the  right  side  through  the  pulmonary  artery,  through  the  capillaries  of 
the  lungs  and  pulmonary  veins,  to  the  left  side  of  the  heart. 

3.  The  portal  circulation,  which  includes  the  portal  vein.  This  vein  is 
formed  by  the  union  of  the  radicles  of  the  gastric,  mesenteric,  and  splenic 
veins,  and  carries  the  blood  directly  into  the  liver,  where  the  vein  divides 
into  a  fine  capillary  plexus,  from  which  the  hepatic  veins  arise ;  these  empty 
into  the  ascending  vena  cava. 

The  Mechanism  of  the  Heart. — The  immediate  cause  of  the  movement 
of  the  blood  through  the  blood-vessels  is  the  alternate  contraction  and  relaxa- 
tion of  the  muscular  walls  of  the  heart,  and  more  especially  the  walls  of  the 
ventricles,  each  of  which  plays  alternately  the  part  of  a  force  pump  and  to 
a  slight  extent  of  a  suction  pump.  The  motive  power  is  furnished  by  the 
heart  itself.  The  contraction  of  any  part  of  the  heart  is  termed  the  systole, 
the  relaxation,  the  diastole;  as  each  side  of  the  heart  has  two  cavities,  the  walls 
of  which  contract  and  relax  in  succession,  it  is  customary  to  speak  of  an 
auricular  systole  and  diastole  and  a  ventricular  systole  and  diastole ;  as  the 
two  sides  are  in  the  same  physiologic  relation  they  contract  and  relax  in  the 
same  periods  of  time. 

Movements  of  the  Heart. — At  each  beat,  during  the  systole,  the  heart 
hardens  and  becomes  shortened  in  its  long  diameter,  its  apex  is  raised  up, 
rotated  on  its  axis  from  left  to  right,  and  pushed  forcibly  against  the  walls 
of  the  chest.  The  impulse  of  the  heart,  observed  about  two  inches  below 
the  nipple  and  one  inch  to  the  sternal  side,  between  the  fifth  and  sixth  ribs, 
is  caused  mainly  by  the  apex  of  the  heart  being  pressed  more  energetically 
against  the  chest  walls. 

The  Cardiac  Cycle. — The  entire  period  of  the  heart's  pulsation  may  be 
divided  into  three  stages,  viz.: 

1.  The  auricular  contraction  and  relaxation. 

2.  The  ventricular  contraction  and  relaxation. 

3.  The  pause  or  period  of  repose  during  which  both  auricles  and  ventricles 
are  at  rest.  These  three  stages  constitute  collectively  a  cardiac  cycle  or  a 
cardiac  revolution. 

The  duration  of  a  cycle,  as  well  as  the  duration  of  its  three  stages,  varies 
in  different  animals  in  accordance  with  the  number  of  cycles  which  recur  in 
a  minute.  In  human  beings  in  adult  fife  there  are  about  72  cycles  to  the 
minute;  the  average  duration  is,  therefore,  0.80  sec.  From  this  it  follows 
that  the  time  occupied  by  any  one  of  the  three  stages  must  be  extremely  short 


120  HUMAN  PHYSIOLOGY. 

and  difficult  of  determination.  From  experiments  on  animals  and  from  obser- 
vations made  on  human  beings,  the  following  estimates  have  been  made  and 
accepted  as  approximately  correct  for  human  beings: 

i.  The  auricular  systole — 0.16  sec;  the  auricular  diastole,  0.64  sec. 

2.  The  ventricular  systole — 0.32  sec;  the  ventricular  diastole,  0.48  sec. 

3.  The  period  of  rest  for  both  auricles  and  ventricles — 0.32  sec. 

The  Action  of  the  Valves. — The  forward  movement  of  the  blood  is 
permitted  and  regurgitation  prevented  by  the  alternate  action  of  the  auriculo- 
ventricular  and  semilunar  valves.  As  a  point  of  departure  for  a  consideration 
of  the  action  of  these  valves  and  their  relation  to  the  systole  and  diastole  of 
the  heart,  the  close  of  the  ventricular  systole  may  be  selected .  At  this  moment, 
the  semilunar  valves,  which  during  the  systole,  are  directed  outward  by  the 
blood  current  are  now  suddenly  and  completely  closed  by  the  pressure  of 
the  blood  in  the  aorta  and  pulmonary  artery.  Regurgitation  into  the 
ventricles  is  thus  prevented. 

During  the  ventricular  systole,  the  relaxed  auricles  are  filling  with  blood. 
With  the  ventricular  diastole  this  blood  or  its  equivalent  flows  into  the  re- 
laxed and  easily  distensible  ventricles  until  both  auricles  and  ventricles  are 
nearly  filled.  The  tricuspid  and  mitral  valves  which  are  hanging  down  into 
the  ventricular  cavities,  are  now  floated  up  by  currents  of  blood  welling  up 
behind  them  until  they  are  nearly  closed.  The  auricles  now  suddenly 
contract,  forcing  their  contained  volumes  into  the  ventricles  which  become 
fully  distended. 

With  the  cessation  of  the  auricular  systole,  the  ventricular  systole  begins. 
If  the  blood  is  not  to  be  returned  to  the  auricles  the  tricuspid  and  mitral 
valves  must  be  instantly  and  completely  closed.  This  is  accomplished  by 
the  upward  pressure  of  the  blood  which  brings  their  free  edges  in  close 
apposition.  Reversal  in  the  position  of  these  valves  is  prevented  by  the  con- 
traction of  the  papillary  muscles  which  exert  a  traction  on  their  under  sur- 
faces and  edges  and  hold  them  steady. 

The  blood  now  confined  in  the  ventricles  between  the  closed  auriculo- 
ventricular  and  semilunar  valves  is  subjected  to  pressure  on  all  sides;  as  the 
pressure  rises  proportionately  to  the  vigor  of  the  contraction  there  comes  a 
moment  when  the  intra-ventricular  pressure  exceeds  that  in  the  aorta  and 
pulmonary  artery;  at  once  the  semilunar  valves  are  thrown  open  and  the  blood 
discharged.  Both  contraction  and  outflow  continue  until  the  ventricles  are 
practically  empty,  when  relaxation  sets  in  attended  by  a  rapid  fall  of  pressure, 
under  the  influence  of  the  positive  pressure  of  the  blood  in  the  aorta  and 
pulmonary  artery,  the  semilunar  valves  are  again  closed.  The  accumu- 
lation of  blood  in  the  auricles,  attended  by  a  rise  in  pressure,  again  forces 


CIRCULATION    OF    THE   BLOOD.  121 

the  tricuspid  and  mitral  valves  open.     With  these  events  the  cardiac  cycle 
is  again  completed. 

Sounds  of  the  Heart. — If  the  ear  be  placed  over  the  cardiac  region,  two 
distinct  sounds  are  heard  during  each  revolution  of  the  heart,  closely  following 
each  other,  and  which  differ  in  character. 

The  sound  coinciding  with  the  systole  in  point  of  time — the  first  sound — 
is  prolonged  and  dull,  and  caused  by  the  closure  and  vibration  of  the  auric- 
uloventricular  valves,  the  contraction  of  the  walls  of  the  ventricles,  and  the 
apex-beat;  the  second  sound,  occurring  during  the  diastole,  is  short  and  sharp, 
and  caused  by  the  closure  of  the  semilunar  valves. 

The  frequency  of  the  heart's  action  varies  at  different  periods  of  life, 
but  in  the  adult  male  it  beats  about  seventy-two  times  a  minute.  It  is  in- 
fluenced by  age,  exercise,  posture,  digestion,  etc. 

Age. — Before  birth,  the  number  of  pulsations  a  minute  averages 140 

During  the  first  year  it  diminishes  to 128 

During  the  third  year  it  dimishes  to 95 

From  the  eighth  to  the  fourteenth  year  averages 84 

In  adult  life  the  average  is 72 

Exercise  and  digestion  increase  the  frequency  of  the  heart's  action. 

Posture  influences  the  number  of  pulsations  a  miruite;  in  the  male,  standing, 
the  average  is  81;  sitting,  71;  lying,  66 — independent,  for  the  most  part,  of 
muscular  effort. 

The  Blood  Supply  to  the  Heart. — The  nutrition  of  the  heart,  its  contrac- 
tility, the  force  and  frequency  of  the  beat  are  dependent  on  and  maintained 
by  the  introduction  of  arterialized  blood  into  and  the  removal  of  waste 
products  from  its  tissue.  This  is  accomplished  by  the  coronary  arteries  and 
the  coronary  veins.  The  arteries  and  veins  on  the  surface  of  the  heart  are 
known  as  the  extra-mural  arteries  and  veins;  those  which  are  found  in  the 
substance  of  the  heart  are  known  as  intra- mural  arteries  and  veins.  During 
the  time  of  the  systole  the  extra-mural  arteries  are  filled  with  blood  from 
the  aorta;  during  the  time  of  the  diastole,  the  blood  flows  into  the  intra- 
mural arteries  and  capillaries,  furnishing  to  the  muscle  cells  an  additional 
supply  of  nutritive  materials  and  receiving  products  of  waste;  at  the  succeed- 
ing systole  the  venous  blood  is  driven  from  the  intra-mural  into  the  extra- 
mural veins  and  so  into  the  right  auricle. 

The  Causation  of  the  Heart  Beat. — From  the  fact  that  the  heart  will 
continue  to  beat  for  a  variable  length  of  time  after  removal  from  the  body 


122  HUMAN  PHYSIOLOGY. 

(the  time  varying  with  the  species  of  animal  from  which  it  has  been  obtained) 
it  is  evident  that  the  beat  is  independent  of  the  central  nerve  system. 

The  fundamental  condition  for  the  continuance  of  the  beat  is  the  main- 
tenance of  the  irritability.  So  long  as  this  persists  the  heart  will  respond  to 
its  appropriate  stimulus.  The  irritability  of  the  heart  within  the  body  is 
dependent  on  the  supply  of  blood  coming  through  its  nutrient  vessels  or 
flowing  through  its  cavities.  Outside  the  body,  the  irritability  can  be  main- 
tained for  some  hours  by  similar  methods. 

The  Nature  of  the  Stimulus. — The  presence  of  nerve  cells  in  the  walls  of 
the  heart,  their  relation  to  the  muscle  cells,  the  pronounced  activity  of  the 
sinus  of  the  frog  heart  where  they  are  very  abundant;  the  feeble  activity  of 
the  apex  where  they  are  absent  gave  rise  to  the  idea  that  the  stimulus  is  a 
nerve  impulse  rhythmically  and  automatically  discharged  by  these  nerve 
cells.  This  view  is  no  longer  entertained.  It  has  been  demonstrated  that 
portions  of  the  heart  muscle,  that  do  not  contain  nerve  cells,  will  continue 
to  exhibit  rhythmic  contraction  for  some  hours  if  supplied  with  oxygenated 
and  defibrinated  blood;  that  the  embryonic  heart  contracts  rhythmically 
before  nerve  cells  have  migrated  to  its  walls. 

The  stimulus  therefore  evidently  arises  within  the  heart  muscle.  In  other 
words,  it  is  myogenic  and  not  neurogenic  in  origin.  The  stimulus  is  now 
believed  to  be  chemic  in  character  and  due  to  a  reaction  between  the  in- 
organic salts  in  the  muscle  cells  and  those  in  lymph  by  which  they  are  sur- 
rounded. 

The  Influence  of  the  Central  Nerve  System  on  the  Action  of  the 
Heart. — Though  the  heart  beat  is  independent  of  the  central  nerve  system, 
it  is  to  a  considerable  extent  modified  by  it  either  in  the  way  of  acceleration 
or  inhibition.  In  all  classes  of  animals  the  heart  not  only  contains  localized 
collections  of  nerve  cells,  but  is  also  connected  with  the  central  nerve  system 
by  two  sets  of  nerve  fibers. 

In  the  frog  heart  a  group  of  nerve  cells  is  found  in  the  sinus  at  its  junction 
with  the  auricle,  and  known  as  the  crescent  or  ganglion  of  Remak;  a  second 
group  is  found  at  the  base  of  the  ventricle  on  its  anterior  aspect  and  known 
as  the  ganglion  of  Bidder;  a  third  group  is  found  in  the  auricular  septum, 
known  as  the  septal  ganglion,  or  the  ganglion  of  Ludwig.  These  cells  were 
formerly  regarded  as  the  source  of  the  stimuli  for  the  heart's  contraction. 
This  view  is  no  longer  entertained. 

In  the  dog  and  the  mammalian  heart  generally,  the  nerve  cells  though 
present  are  not  arranged  in  such  definite  groups,  but  are  distributed  in  the 
terminations  of  the  venae  cavae,  pulmonary  veins,  the  walls  of  the  auricles  and 
in  the  neighborhood  of  the  base  of  the  ventricles. 


CIRCULATION   OF   THE   BLOOD.  1 23 

The  nerves  which  connect  the  heart  with  the  central  nerve  system  are  the 
sympathetic  and  the  pneumogastric,  or  vagus. 

The  sympathetic  nerves  are  derived  mainly  from  the  ganglion  Stella  turn. 
The  cells  of  this  ganglion,  however,  are  in  relation  with  nerve  fibers  which 
emerge  from  the  spinal  cord  in  the  second  and  third  thoracic  nerves,  and 
which  have  their  origin  in  cells  located  most  probably  in  the  medulla 
oblongata. 

Stimulation  of  the  sympathetic  fibers  beyond  the  ganglion  stellatum, 
is  followed  by  an  increase  in  the  rate  and  sometimes  by  an  increase  in  the 
force  of  the  heart  beat.  For  this  reason  the  sympathetic  is  said  to  exert  an 
accelerator  and  an  augmentor  influence  on  the  heart  beat.  The  center  from 
which  the  nerve  impulses  physiologically  arise  is  known  as  the  cardio  acceler- 
ator center. 

The  pneumogastric,  or  vagus  nerve,  close  to  its  connection  with  the  medulla 
oblongata,  receives  motor  nerves  from  the  spinal  accessory.  It  also  contains 
efferent  fibers,  which  come  direct  from  the  medulla.  It  then  passes  down  the 
neck  and  enters  the  thorax.  Some  of  its  fibers  join  the  cardiac  plexus  and 
by  this  route  reach  the  heart.  Experimental  evidence  indicates  that  the 
terminal  fibers  of  the  vagus  arborize  around  the  nerve  cells  in  the  heart  wall. 
Feeble  stimulation  of  the  trunk  of  the  vagus  is  followed  by  a  diminution  in  the 
rate  of  the  beat;  strong  stimulation  is  followed  by  complete  cessation  or  in- 
hibition of  the  heart  beat,  the  organ  coming  to  rest  in  the  condition  of  diastole. 
Division  of  both  vagi,  in  the  dog,  at  a  time  when  the  heart  is  beating  nor- 
mally, is  followed  by  a  considerable  increase  in  the  frequency  of  the  beat. 
For  these  reasons  the  vagus  nerve  is  said  to  have  an  inhibitor  or  restraining 
influence  on  the  rate  of  the  heart  beat. 


ARTERIES. 

The  arteries  are  a  series  of  branching  tubes  conveying  blood  to  all  por- 
tions of  the  body.     They  are  composed  of  three  coats: 

1.  External,  formed  of  areolar  and  elastic  tissue. 

2.  Middle,  contains  both  elastic  and  muscle  fibers,  arranged  transversely 
to  the  long  axis  of  the  artery.  The  elastic  tissue  is  more  abundant  in  the 
larger  vessels,  the  muscular  in  the  smaller. 

3.  Internal,  composed  of  a  thin,  homogeneous  membrane,  covered  with  a 
layer  of  elongated  endothelial  cells. 

The  arteries  possess  both  elasticity  and  contractility. 

The  property  of  elasticity  allows  the  arteries  already  full  to  accommodate 
themselves  to  the  incoming  amount  of  blood,  and  to  convert  the  intermit- 


124  HUMAN  PHYSIOLOGY. 

tent  acceleration  of  blood  in  the  large  vessels  into  a  steady  and  continuous 
stream  in  the  capillaries. 

The  contractility  of  the  smaller  vessels  equalizes  the  current  of  blood,  regu- 
lates the  amount  going  to  each  capillary  area,  and  promotes  the  onward  flow 
of  blood. 

Blood  Pressure. — The  immediate  cause  of  the  movement  of  the  blood 
from  the  beginning  of  the  aorta,  through  the  arteries,  capillaries  and  veins, 
to  the  right  side  of  the  heart,  is  a  difference  of  pressure  between  these  two 
points.  A  corresponding  difference  of  pressure  exists  between  the  beginning 
of  the  pulmonary  artery  and  the  left  side  of  the  heart.  To  this  pressure  the 
term  blood  pressure  is  given  and  may  be  defined  as  the  pressure  exerted 
laterally  by  the  moving  blood  stream  against  the  walls  of  the  arteries,  capil- 
laries and  veins.  That  there  is  such  a  pressure  different  in  amount. in  each 
of  these  three  divisions  of  the  vascular  apparatus  is  evident  from  the  results 
which  follow  division  of  an  artery  or  a  vein  of  corresponding  size.  When 
an  artery  is  divided  the  blood  spurts  from  the  opening  for  a  considerable 
distance  and  with  considerable  velocity.  When  a  vein  is  divided  the  blood 
as  a  rule  merely  wells  out  of  the  opening  and  with  but  slight  momentum. 
These  results  indicate  that  the  blood  exerts  a  greater  pressure  in  the  arteries 
than  in  the  veins.  Experimentally  it  has  been  shown  that  the  pressure  is 
greatest  in  the  aorta,  less  in  the  capillaries,  and  least  in  the  veins.  The 
pressure  in  the  aorta  expressed  in  millimeters  of  mercury  is  about  160,  in 
the  capillaries  35  to  20,  and  in  the  veins  from  20  to  o  or  less  at  the  termin- 
ations of  the  venae  cavae. 

The  causes  of  the  blood  pressure  are  first,  the  driving  power  of  the  heart, 
and  second,  the  resistance  offered  by  the  walls  of  the  blood-vessels  to  the  flow 
of  blood  through  them.  Owing  to  this  resistance,  the  blood  has  accumu- 
lated and  in  consequence  the  whole  system  has  become  distended  by  the 
lateral  pressure.  The  largest  part  of  the  resistance,  however,  is  found  at 
the  periphery  of  the  arterial  system  and  is  partly  the  cause  of  the  high  pres- 
sure in  the  arteries. 

The  arterial  pressure  is  increased  or  decreased  by  influences  which  act 
upon  the  heart  or  upon  this  peripheral  resistance. 

If  while  the  force  of  the  heart  remains  the  same,  the  rate  increases,  thus 
increasing  the  volume  of  blood  in  the  arteries,  the  pressure  rises.  If  the 
rate  remains  the  same,  but  the  volume  of  blood  discharged  increases,  the 
pressure  will  also  rise.  If  the  peripheral  resistance  is  increased  by  contrac- 
tion of  the  arterioles  the  pressure  rapidly  rises.  On  the  contrary,  a  diminu- 
tion in  the  rate  and  force  of  the  heart  or  a  diminution  in  peripheral  resistance 
by  a  dilatation  of  the  arteries  cause  a  fall  in  pressure. 


CIRCULATION    OF    THE    BLOOD.  1 25 

The  Pulse. — The  pulse  may  be  defined  as  a  periodic  expansion  and  recoil 
of  the  arterial  system.  The  expansion  is  caused  by  the  ejection  into  the 
arteries  of  a  volume  of  blood  during  the  systole;  the  recoil  is  due  to  the 
reaction  of  the  arterial  walls  on  the  blood  driving  it  forward  into  and  through 
the  capillaries,  during  the  diastole. 

At  the  close  of  the  cardiac  diastole  the  arteries  are  full  of  blood  and  con- 
siderably distended.  During  the  occurrence  of  the  succeeding  systole,  the 
incoming  volume  of  blood  is  accommodated  by  a  movement  forward  of  a 
portion  of  the  general  blood  volume  into  the  capillaries  and  a  further  disten- 
tion of  the  arteries.  The  distention  naturally  begins  at  the  beginning  of  the 
aorta.  As  the  blood  continues  to  be  discharged  from  the  heart,  adjoining 
segments  of  the  aorta  expand  in  quick  succession  and  by  the  end  of  the  sys- 
tole the  expansion  has  travelled  over  the  arterial  system  as  far  as  the  capil- 
laries. This  expansion  movement  which  passes  over  the  arterial  system  in 
the  form  of  a  wave  is  known  as  the  pulse  wave,  or  the  pulse.  It  is  this  alter- 
nate expansion  and  recoil  which  is  perceived  by  the  finger  when  placed  over 
the  course  of  an  artery.  The  artery  best  adapted  for  this  purpose  is  the 
radial  as  it  passes  across  the  wrist  joint. 

The  velocity  with  which  the  blood  flows  in  the  arteries  diminishes  from 
the  heart  to  the  capillaries,  owing  to  an  enlargement  in  the  united  sectional 
area  of  the  vessels;  the  velocity  increases  from  the  capillaries  toward  the 
heart  for  the  opposite  reason.  The  blood  moves  most  rapidly  in  the  large 
vessels,  and  especially  under  the  influence  of  the  ventricular  systole.  From 
experiments  on  animals,  it  has  been  estimated  to  move  in  the  carotid  of  man 
at  the  rate  of  sixteen  inches  a  second,  and  in  the  large  veins  at  the  rate  of 
four  inches  a  second. 

The  caliber  of  the  peripheral  arteries  is  regulated  by  the  vasomotor 
nerves,  which  have  their  origin  in  the  gray  matter  of  the  medulla  oblongata. 
They  issue  from  the  spinal  cord  through  the  anterior  roots  of  spinal  nerves 
from  the  second  thoracic  to  the  third  or  fourth  lumbar  nerve,  pass  through 
the  sympathetic  ganglia  in  different  situations  around  the  nerve  cells  of 
which  they  arborize.  From  the  ganglion  cells  new  fibers  arise  which  ulti- 
mately are  distributed  to  the  coats  of  the  blood-vessels.  They  exert  at 
different  times  a  constricting  and  a  dilating  action  upon  the  vessels,  thus 
keeping  up  the  arterial  tonus  and  the  average  blood  pressure. 

Capillaries. — The  capillaries  constitute  a  network  of  vessels  of  micro- 
scopic size,  which  distribute  the  blood  to  the  inmost  recesses  of  the  tissues, 
inosculating  with  the  arteries  on  the  one  hand  and  the  veins  on  the  other; 
they  branch  and  communicate  in  every  possible  direction. 


126  HUMAN  PHYSIOLOGY. 

The  diameter  of  a  capillary  vessel  varies  from  gVoo  to  ^oVo  °f  an  inch; 
the  walls  of  these  vessels  consist  of  a  delicate,  homogeneous  membrane, 
2 offo¥  °f  an  m°h  m  thickness,  Hned  by  flattened,  elongated,  and  endothelial 
cells,  between  which,  here  and  there,  are  observed  stomata.  In  many  capil- 
laries the  wall  consists  only  of  the  endothelial  cells. 

The  rate  of  movement  in  the  capillary  vessels  is  estimated  at  one  inch  in 
thirty  seconds. 

In  the  capillary  current  the  red  corpuscles  may  be  seen  hurrying  down  the 
center  of  the  stream,  while  the  white  corpuscles  in  the  still  layer  adhere  to  the 
walls  of  the  vessel,  and  at  times  can  be  seen  to  pass  through  the  walls  of  the 
vessel  by  ameboid  movements. 

The  function  of  the  capillary  blood-vessel  is  to  permit  of  the  passage  of 
the  nutritive  materials  of  the  blood  out  into  the  tissue-spaces  and  the  passage 
of  waste  products  from  the  tissue-spaces  into  the  blood. 

The  passage  of  the  blood  through  the  capillaries  is  mainly  due  to  the 
force  of  the  ventricular  systole  and  the  elasticity  of  the  arteries;  but  it  is 
possibly  also  aided  by  a  power  resident  in  the  capillaries  themselves,  the  result 
of  a  vital  relation  between  the  blood  and  the  tissues. 

The  veins  are  the  vessels  which  return  the  blood  to  the  heart;  they  have 
their  origin  in  the  venous  radicles,  and  as  they  approach  the  heart  converge 
to  form  larger  trunks,  and  terminate  finally  in  the  venae  cavse. 

They  possess  threecoats — 
i.  External,  made  up  of  areolar  tissue. 

2.  Middle,   composed    of   non -striated    muscle-fibers;   yellow,   elastic    and 
fibrous  tissue. 

3.  Internal,  an  endothelial  membrane  similar  to  that  of  the  arteries. 

Veins  are  distinguished  by  the  possession  of  valves  throughout  their 
course,  which  are  arranged  in  pairs,  and  formed  by  a  reflection  of  the  in- 
ternal coat,  strengthened  by  fibrous  tissues;  they  always  look  toward  the 
heart,  and  when  closed  prevent  a  reflux  of  blood  in  the  veins.  Valves  are 
most  numerous  in  the  veins  of  the  extremities,  but  are  entirely  absent  in 
many  others. 

The  onward  flow  of  blood  in  the  veins  is  mainly  due  to  the  action  of 
the  heart,  but  is  assisted  by  the  contraction  of  the  voluntary  muscles  and  the 
force  of  respiration. 

Muscular  contraction,  which  is  intermittent,  aids  the  flow  of  blood  in  the 
veins  by  compressing  them.  As  regurgitation  is  prevented  by  the  closure 
of  the  valves,  the  blood  is  forced  onward  toward  the  heart. 

During  the  movement  of  inspiration  the  thorax  is  enlarged  in  all  its  diam- 
eters, and  the  pressure  on  its  contents  at  once  diminishes.     Under  these 


RESPIRATION.  1 27 

circumstances  a  negative  pressure  is  established  in  the  thorax  in  consequence 
of  which  the  blood  is  caused  to  flow  with  increased  rapidity  and  volume 
toward  the  heart. 

Venous  Pressure. — As  the  force  of  the  heart-beat  is  nearly  expended 
in  driving  the  blood  through  the  capillaries,  the  pressure  in  the  venous 
system  is  not  very  marked,  not  amounting  in  the  jugular  vein  of  a  dog  to 
more  than  ^  that  of  the  carotid  artery. 

The  time  required  for  a  complete  circulation  of  the  blood  throughout  the 
vascular  system  has  been  estimated  to  be  from  twenty  to  thirty  seconds, 
while  for  the  entire  volume  of  blood  to  pass  through  the  heart  thirty-three 
pulsations  would  be  required,  occupying  about  thirty-five  seconds. 

The  forces  keeping  the  blood  in  circulation  are: 

1.  Action  of  the  heart. 

2.  Elasticity  of  the  arteries. 

3.  Capillary  force. 

4.  Contraction  of  the  voluntary  muscles  upon  the  veins. 

5.  Respiratory  movements. 


RESPIRATION. 

Respiration  is  the  process  by  which  oxygen  is  absorbed  into  the  blood 
and  carbonic  acid  exhaled.  The  assimilation  of  the  oxygen  and  the  evolution 
of  carbon  dioxid  takes  place  in  the  tissues  as  a  part  of  the  general  nutritive 
process,  the  blood  and  respiratory  apparatus  constituting  the  media  by 
means  of  which  the  interchange  of  gases  is  accomplished. 

The  respiratory  apparatus  consists  of  a  larynx,  trachea,  and  lungs. 

The  larynx  is  composed  of  firm  cartilages,  united  by  ligaments  and 
muscles.  Running  anteroposteriorly  across  the  upper  opening  are  four 
ligamentous  bands — the  two  superior  or  false  vocal  bands,  and  the  two 
inferior  or  true  vocal  bands — formed  by  folds  of  the  mucous  membrane. 
They  are  attached  anteriorly  to  the  thyroid  cartilages  and  posteriorly  to  the 
arytenoid  cartilages,  and  are  capable  of  being  separated  by  the  contraction 
of  the  posterior  crico-arytenoid  muscles,  so  as  to  admit  the  passage  of  air 
into  and  from  the  lungs. 

The  trachea  is  a  tube  from  four  to  five  inches  in  length,  |  of  an  inch 
in  diameter,  extending  from  the  cricoid  cartilage  of  the  larynx  to  the  third 
dorsal  vertebra,  where  it  divides  into  the  right  and  left  bronchi.  It  is  com- 
posed of  a  series  of  cartilaginous  rings,  which  extend  about  two  thirds  around 


128 


HUMAN  PHYSIOLOGY. 


its  circumference,  the  posterior  third  being  occupied  by  fibrous  tissue  and 
non-striated  muscle-fibers,  which  are  capable  of  diminishing  its  caliber. 

The  trachea  is  covered  externally  by  a  tough,  fibro-elastic  membrane, 
and  internally  by  mucous  membiane,  lined  by  columnar,  ciliated,  epithelial 
cells.  The  cilia  are  always  waving  from  within  outward.  When  the  two 
bronchi  enter  the  lungs,  they  divide  and  subdivide  into  numerous  smaller 
branches,  which  penetrate  the  lungs  in  every  direction  until  they  finally 
terminate  in  the  pulmonary  lobules. 

As  the  bronchial  tubes  become  smaller 
their  walls  become  thinner;  the  cartilagin- 
ous rings  disappear,  but  are  replaced  by 
irregular  angular  plates  of  cartilage;  when 
the  tube  becomes  less  than  ■£$  °f  an  mcri 
in  diameter,  they  wholly  disappear,  and  the 
fibrous  and  mucous  coats  blend,  forming  a 
delicate  elastic  membrane,  with  circular 
muscle-fibers. 


The  lungs  occupy  the  cavity  of  the 
thorax,  are  conic  in  shape,  of  a  pink  color 
and  a  spongy  texture.  They  are  composed 
of  a  great  number  of  distinct  lobules  (the 
pulmonary  lobules),  connected  by  interlob- 
ular connective  tissue.  These  lobules  vary 
in  size,  are  of  an  oblong  shape,  and  are  com- 
posed of  the  ultimate  ramifications  of  the 
bronchial  tubes,  within  which  are  contained 
the  air-vesicles  or  cells.  The  walls  of  the 
air-vesicles,  exceedingly  thin  and  delicate, 
are  lined  internally  by  a  layer  of  tessellated 
epithelium,  externally  covered  by  elastic 
fibers,  which  give  the  lungs  their  elasticity 
and  distensibility. 
The  venous  blood  is  distributed  to  the  lungs  for  aeration  by  the  pulmonary 
artery,  the  terminal  branches  of  which  form  a  rich  plexus  of  capillary  vessels 
surrounding  the  air-cells;  the  air  and  blood  are  thus  brought  into  intimate 
relationship,  being  separated  only  by  the  delicate  walls  of  the  air-cells  and 
capillaries. 

The  Thorax. — The  thorax  in  which  the  respiratory  organs  are  lodged,  is 
of  a  conic  shape,  having  its  apex  directed  upward,  its  base  downward.  Its 
framework  is  formed  posteriorly  by  the  spinal  column,  anteriorly  by  the  ster- 


Fig.  18. — Diagram  of  the  Res- 
piratory Organs. 

The  windpipe,  leading  down 
from  the  larynx,  is  seen  to  branch 
into  two  large  bronchi,  which  sub- 
divide after  they  enter  their  re- 
spective lungs. 


RESPIRATION.  1 29 

The  Thorax. — The  thorax  in  which  the  respiratory  organs  are  lodged,  is 
of  a  conic  shape,  having  its  apex  directed  upward,  its  base  downward.  Its 
framework  is  formed  posteriorly  by  the  spinal  column,  anteriorly  by  the  ster- 
num, and  laterally  by  the  ribs  and  costal  cartilages.  Between  and  over  the 
ribs  lie  muscles,  fascia,  and  skin,  above,  the  thorax  is  completely  closed  by 
the  structures  passing  into  it  and  by  the  cervical  fascia  and  skin;  below,  it 
is  closed  by  the  diaphragm.     It  is,  therefore,  an  air-tight  cavity. 

The  Pleura. — Each  lung  is  surrounded  by  a  closed  serous  membrane 
(the  pleura),  one  layer  of  which  (the  visceral)  is  reflected  over  the  lung;  the 
other  (the  parietal),  reflected  over  the  wall  of  the  thorax;  between  the  two 
layers  is  a  small  amount  of  fluid,  which  prevents  friction  during  the  play 
of  the  lungs  in  respiration. 

Owing  to  the  elastic  tissue  which  is  present  in  the  lungs,  they  are  very 
readily  distensible;  so  much  so,  indeed,  that  the  pressure  of  the  air  inside 
the  trachea  and  lungs  is  sufficient  to  distend  them  until  they  completely 
fill  all  parts  of  the  thoracic  cavity  not  occupied  by  the  heart  and  great  vessels. 
The  elastic  tissue  endows  them  not  only  with  distensibility,  but  also  with 
the  power  of  elastic  recoil,  by  which  they  are  enabled  to  accommodate  them- 
selves to  all  variations  in  the  size  of  the  thoracic  cavity. 

When  the  chest-walls  recede,  the  air  within  the  lungs  expands  and  presses 
them  against  the  ribs;  when  the  chest-walls  contract,  the  air  being  driven 
out,  the  elastic  tissue  recoils  and  the  lungs  return  to  their  original  condition. 
The  movements  of  the  lungs  are,  therefore,  entirely  passive. 

As  the  capacity  of  the  chest  in  a  state  of  rest  is  greater  than  the  volume 
of  the  lungs  after  they  are  collapsed,  it  is  quite  evident  that  in  the  living 
condition  the  lungs  are  distended  and  in  a  state  of  elastic  tension,  which  is 
greater  or  less  in  proportion  as  the  thoracic  cavity  is  increased  or  diminished 
in  size.  The  elastic  tissue,  always  on  the  stretch,  is  endeavoring  to  pull  the 
visceral  layer  of  the  pleura  away  from  the  parietal  layer,  but  is  antagonized 
by  the  pressure  of  the  air  within  the  air-passages.  This  condition  of  things 
persists  as  long  as  the  thoracic  cavity  remains  air-tight;  but  if  an  opening  be 
made  in  the  thoracic  wall,  the  pressure  of  the  external  air,  which  was  pre- 
viously supported  by  the  practically  rigid  walls  of  the  thorax,  now  presses 
upon  the  lung  with  as  much  force  as  the  air  within  the  lung.  The  two  pres- 
sures being  neutralized,  there  is  nothing  to  prevent  the  elastic  tissue  from  re- 
coiling, driving  the  air  out,  and  collapsing.  The  elastic  tension  of  the  lungs 
can  be  readily  measured  in  man  after  death  by  inseriting  a  manometer  into 
the  trachea.  Upon  opening  the  thorax  and  allowing  the  tissue  to  recoil,  the 
air  passes  upon  the  mercury  and  elevates  it,  the  extent  to  which  it  is  raised 
being  the  index  of  the  pressure.  Hutchinson  calculated  the  pressure  to  be 
one  half  pound  to  the  square  inch  of  lung  surface. 

9 


130  HUMAN  PHYSIOLOGY. 

Respiratory  Movements. — The  movements  of  respiration  are  two  and 
consist  of  an  alternate  expansion  and  recoil  of  the  thorax,  known  as  inspiration 
and  expiration. 

1.  Inspiration  is  an  active  process,  the  result  of  the  expansion  of  the  thorax, 
whereby  the  atmospheric  air  is  introduced  into  the  lungs. 

2.  Expiration  is  a  partially  passive  process,  the  result  of  the  recoil  of  the 
elastic  walls  of  the  thorax,  and  the  recoil  of  the  elastic  tissue  of  the  lungs 
whereby  the  intrapulmonary  air  is  expelled. 

In  inspiration  the  chest  is  enlarged  by  an  increase  in  all  its  diameters — 
viz.: 

1.  The  vertical  is  increased  by  the  contraction  and  descent  of  the  diaphragm. 

2.  The  anteroposterior  and  transverse  diameters  are  increased  by  the  eleva- 
tion and  rotation  of  the  ribs  upon  their  axes. 

In  ordinary  tranquil  inspiration  the  muscles  which  elevate  the  ribs  and 
thrust  the  sternum  forward,  and  so  increase  the  diameters  of  the  chest,  are 
the  external  intercostals,  running  from  above  downward  and  forward;  the 
sternal  portion  of  the  internal  intercostals,  and  the  levator es  costarum. 

In  the  extraordinary  efforts  of  inspiration  certain  auxiliary  muscles  are 
brought  into  play — viz.,  the  sternomastoid,  pectorales,  serratus  magnus — 
which  increase  the  capacity  of  the  thorax  to  its  utmost  limit.  • 

In  expiration  the  diameters  of  the  chest  are  all  diminished — viz. : 

1.  The  vertical,  by  the  ascent  of  the  diaphragm. 

2.  The  anteroposterior,  by  a  depression  of  the  ribs  and  sternum. 

In  ordinary  tranquil  expiration  the  diameters  of  the  thorax  are  diminished 
by  the  recoil  of  the  elastic  tissue  of  the  lungs  and  the  ribs;  but  in  forcible 
expiration  the  muscles  which  depress  the  ribs  and  sternum,  and  thus  further 
diminish  the  diameter  of  the  chest,  are  the  internal,  intercostals,  the  infra- 
costals,  and  the  triangularis  sterni. 

In  the  extraordinary  efforts  of  expiration  certain  auxiliary  muscles  are 
btought  into  play — viz.,  the  abdominal  and  sacrolumbalis  muscles — which 
diminish  the  capacity  of  the  thorax  to  its  utmost  limit. 

Expiration  is  aided  by  the  recoil  of  the  elastic  tissue  of  the  lungs  and 
ribs  and  by  the  pressure  of  the  air. 

Movements  of  the  Glottis. — At  each  inspiration  the  rima  glottidis  is 
dilated  by  a  separation  of  the  vocal  cords,  produced  by  the  contraction  of  the 
crico-arytenoid  muscles,  so  as  freely  to  admit  the  passage  of  air  into  the 
lungs;  in  expiration  they  fall  passively  together,  but  do  not  interfere  with  the 
exit  of  air  from  the  chest. 

Nerve  Mechanism   of  Respiration. — The  movements  of  respiratory 


RESPIRATION.  131 

muscles,  though  capable  of  being  modified  to  a  certain  extent  by  efforts  of 
the  will,  are  of  an  automatic  character,  and  called  forth  by  nerve  impulses 
emanating  from  the  medulla  oblongata.  The  inspiratory  center,  generates 
the  nerve  impulses,  which,  traveling  outward  through  the  phrenic  and  in- 
tercostal nerves,  excite  contractions  of  the  diaphragm  and  intercostal  muscles, 
respectively.  This  center  is  for  the  most  part  automatic  in  its  action,  though 
it  is  capable  of  being  modified  by  impulses  reflected  to  it  through  various 
sensor  nerves. 

This  center  may  be  stimulated: 

r.  Directly,  by  the  condition  of  the  blood.  An  increase  of  carbonic  acid 
or  a  diminution  of  oxygen  in  the  blood  causes  an  acceleration  of  the  re- 
spirator}' movements;  the  reverse  of  these  conditions  causes  a  diminution 
of  the  respirator}*  movements. 

2.  Indirectly,  by  reflex  action.  The  medulla  may  be  excited  to  action 
through  the  pneumogastric  nerve,  by  the  presence  of  carbonic  acid  in  the 
lungs  irritating  its  terminal  filaments;  through  the  fifth  nerve,  by  irritation 
of  the  terminal  branches;  and  through  the  nerves  of  general  sensibilitv. 
In  either  case  this  center  reflects  motor  impulses  to  the  respiratory  muscles 
through  the  phrenic,  intercostal,  inferior  laryngeal,  and  other  nerves. 
Types  of  Respiration. — The  abdominal  type  is  most  marked  in  young 

children,  irrespective  of  sex,  the  respiratory  movements  being  effected  by 

the  diaphragm  and  abdominal  muscles. 

In  the  superior  costal  type,  exhibited  by  the  adult  female,  the  respiratory 

movements  are  more  marked  in  the  upper  part  of  the  chest,  from  the  first 

to  the  seventh  ribs,  permitting  the  uterus  to  ascend  in  the  abdomen  during 

pregnancy  without  interfering  with  respiration. 

In  the  inferior  costal  type,  manifested  by  the  male,  the  movements  are 

largely  produced  by  the  muscles  of  the  lower  portions  of  the  chest,  from  the 

seventh  rib  downward,  assisted  by  the  diaphragm. 

The  respiratory  movements  vary  according  to  age,  sleep,  and  exercise, 

being  most  frequent  in  early  life,  but  averaging  twenty  a  minute  in  adult 

life.     They  are  diminished  by  sleep  and  increased  by  exercise.     There  are 

about  four  pulsations  of  the  heart  to  each  respirator}-  act. 

During  both  inspiration  and  expiration  two  sounds  are  produced:  the  one, 

heard  in  the  thorax,  in  the  trachea,  and  larger  bronchial  tubes,  is  tubular  in 

character;  the  other,  heard  in  the  substance  of  the  lungs,  is  vesicular  in 

character. 

Amount  of  Air  Exchanged  in  Respiration,  and  Capacity  of  Lungs. 

The  tidal  or  breathing  volume  of  air,  that  which  passes  in  and  out  of  the 
lungs  at  each  inspiration  and  expiration,  is  estimated  at  from  twenty  to 
thirty  cubic  inches. 


132  HUMAN  PHYSIOLOGY. 

The  complemental  air  is  that  amount  which  can  be  taken  into  the  lungs 
by  a  forced  inspiration,  in  addition  to  the  ordinary  tidal  volume,  and  amounts 
to  about  no  cubic  inches. 

The  reserve  air  is  that  which  usually  remains  in  the  chest  after  the  ordinary 
efforts  of  expiration,  but  which  can  be  expelled  by  forcible  expiration.  The 
volume  of  reserve  air  is  about  100  cubic  inches. 

The  residual  air  is  that  portion  which  remains  in  the  chest  and  cannot 
be  expelled  after  the  most  forcible  expiratory  efforts,  and  which  amounts, 
according  to  Dr.  Hutchinson,  to  about  100  cubic  inches. 

The  vital  capacity  of  the  chest  indicates  the  amount  of  air  that  can  be 
forcibly  expelled  from  the  lungs  after  the  deepest  possible  inspiration,  and 
is  an  index  of  an  individual's  power  of  breathing  in  disease  and  during  pro- 
longed severe  exercise.  The  combined  amount  of  the  tidal,  the  comple- 
mental, and  the  reserve  air,  230  cubic  inches,  represents  the  vital  capacity 
of  an  individual  five  feet  seven  inches  in  height.  The  vital  capacity  varies 
chiefly  with  stature.  It  is  increased  eight  cubic  inches  for  every  inch  in 
height  above  this  standard,  and  diminishes  eight  cubic  inches  for  each  inch 
below  it. 

The  tidal  volume  of  air  is  carried  only  into  the  trachea  and  large  bronchial 
tubes  by  the  inspiratory  movements.  It  reaches  the  deeper  portions  of  the 
lungs  in  obedience  to  the  law  of  diffusion  of  gases,  which  is  inversely  pro- 
portionate to  the  square  root  of  their  densities. 

The  ciliary  action  of  the  columnar  cells  lining  the  broncial  tubes  also 
assists  in  the  interchange  of  air  and  carbonic  acid. 

The  entire  volume  of  air  passing  in  and  out  of  the  thorax  in  twenty-four 
hours  is  subject  to  great  variation,  but  can  be  readily  estimated  from  the 
tidal  volume  and  the  number  of  respirations  a  minute.  Assuming  that  an 
individual  takes  into  the  chest  twenty  cubic  inches  at  each  inspiration,  and 
breathes  eighteen  times  a  minute,  in  twenty-four  hours  there  would  pass  in 
and  out  of  the  lungs  518,400  cubic  inches,  or  300  cubic  feet. 

Chemistry  of  Respiration. — As  the  inspired  air  undergoes  a  change  in 
composition  during  its  stay  in  the  lungs  which  renders  it  unfit  for  further 
respiration,  it  becomes  requisite,  for  the  correct  understanding  of  respiration, 
to  ascertain  the  composition  of  both  inspired  and  expired  air. 

Composition  of  Air. — Chemic  analysis  has  shown  that  every  100  volumes 
of  air  contain  20.81  volumes  of  oxygen,  70.19  volumes  of  nitrogen,  and  0.03 
volume  of  carbonic  acid.  Aqueous  vapor  is  also  present,  though  the  quan- 
tity is  variable.     The  higher  the  temperature,  the  greater  the  amount. 


RESPIRATION.  133 

The  changes  in  the  air  affected  by  respiration  are: 
Loss  of  oxygen,  to  the  extent  of  five  cubic  inches  per  100  of  air,  or  one  in 

twenty. 
Gain  of  carbonic  acid,  to  the  extent  of  4.66  cubic  inches  per  100  of  air,  or 

0.93  inch  in  twenty. 
Increase  of  water-vapor  and  organic  matter. 
Elevation  of  temperature. 
Increase,  and  at  times  decrease,  of  nitrogen. 
Gain  of  ammonia. 

The  total  quantity  of  oxygen  withdrawn  from  the  air  and  consumed  by  the 
body  in  twenty-four  hours  amounts  to  fifteen  cubic  feet,  and  can  be  readily 
estimated  from  the  amount  consumed  at  each  respiration.  Assuming  that 
one  cubic  inch  of  oxygen  remains  in  the  lungs  at  each  respiration,  in  one  hour 
there  are  consumed  1080  cubic  inches,  and  in  twenty-four  hours  25,920  cubic 
inches,  or  fifteen  cubic  feet,  weighing  eighteen  ounces.  To  obtain  this 
quantity,  300  cubic  feet  of  air  are  necessary. 

The  quantity  of  oxygen  consumed  daily  is  subject  to  considerable  vari- 
ations. It  is  increased  by  exercise,  digestion,  and  lowered  temperature,  and 
decreased  by  the  opposite  conditions. 

The  quantity  of  carbonic  acid  exhaled  in  twenty-four  hours  varies  greatly. 
If  can  be  estimated  in  the  same  way.  Assuming  that  an  individual  ex- 
hales 0.93  +  cubic  inch  at  each  respiration,  in  one  hour  there  are  eliminated 
1008  cubic  inches,  and  in  twenty-four  24,192  cubic  inches,  or  fourteen  cubic 
feet,  containing  seven  ounces  of  pure  carbon. 

The  exhalation  of  carbonic  acid  is  increased  by  muscular  exercise,  nitrog- 
enous food,  tea,  coffee,  and  rice,  age,  and  by  muscular  development;  decreased 
by  a  lowering  of  temperature,  repose,  gin  and  brandy,  and  a  dry  condition 
of  the  air. 

As  there  is  always  more  oxygen  consumed  than  carbonic  acid  exhaled, 
and  as  oxygen  unites  with  carbon  to  form  an  equal  volume  of  carbonic  acid, 
it  is  evident  that  a  certain  quantity  of  oxygen  disappears  within  the  body. 
In  all  probability  it  unites  with  the  surplus  hydrogen  of  the  food  to  form 
water. 

The  amount  of  water  vapor  which  passes  out  of  the  body  with  the  expired 
air  is  estimated  at  from  one  to  two  pounds. 

The  organic  matter,  though  slight  in  amount,  gives  the  odor  to  the  breath. 
In  a  room  with  defective  ventilation  the  organic  matter  accumulates  and  gives 
rise  to  headache,  nausea,  drowsiness,  etc.  Long-continued  breathing  of 
such  air  produces  general  ill  health.  It  is  not  so  much  the  presenc  of  C02 
in  increased  amount  as  the  presence  of  organic  matter  which  necessitates 
thorough  ventilation. 


134  HUMAN  PHYSIOLOGY. 

Condition  of  the  Gases  in  the  Blood. 

Oxygen  is  absorbed  from  the  lungs  into  the  arterial  blood  by  the  coloring- 
matter,  hemoglobin,  with  which  it  exists  in  a  state  of  loose  combination,  and 
is  disengaged  in  its  passage  through  the  capillaries. 

Carbonic  acid,  arising  in  the  tissues,  is  absorbed  into  the  blood  in  conse- 
quence of  its  alkalinity,  where  it  exists  in  a  state  of  simple  solution  and  also 
in  a  state  of  feeble  combination  with  the  carbonates,  soda  and  potassa, 
forming  the  bicarbonates.  It  is  liberated  as  the  blood  flows  through  the 
capillaries  of  the  lungs. 

Nitrogen  is  simply  held  in  solution  in  the  plasma. 

Exchange  of  Gases  in  the  Air-cells. — From  the  difference  in  ten- 
sion of  the  oxygen  in  the  air-cells  (27.44  mm.  of  Hg)  and  of  the  oxygen  in 
the  venous  blood  (22  mm.  Hg),  and  from  the  difference  of  the  carbonic  acid 
tension  in  the  venous  blood  (41  mm.  Hg)  and  in  the  air-cells  (27  mm.  Hg), 
it  might  be  concluded  that  the  passage  of  the  gases  is  due  solely  to  pressure. 
The  absorption  of  oxygen,  however,  does  not  follow  absolutely  the  law  of 
pressure ;  that  chemic  processes  are  involved  is  shown  by  the  union  of  oxygen 
with  the  hemoglobin  of  the  blood  corpuscles.  The  exhalation  of  C02  is  also 
partly  a  chemic  process,  as  it  has  been  shown  that  the  quantity  excreted  is 
greatly  increased  when  oxygen  is  simultaneously  absorbed.  Oxygen  not 
only  favors  the  exhalation  of  loosely  combined  C02,  but  favors  the  expulsion 
of  that  which  can  be  excreted  only  by  the  addition  of  acids  to  the  blood. 

Changes  in  the  Blood  during  Respiration. 

As  the  blood  passes  through  the  lungs  it  is  changed  in  color,  from  the 
bluish-red  of  venous  blood  to  the  scarlet-red  of  arterial  blood. 
It  gains  oxygen  and  loses  carbonic  acid. 
Its  coagulability  is  increased.     Its  temperature  is  diminished. 

Asphyxia. — If  the  supply  of  oxygen  to  the  lungs  be  diminished  and  the 
carbonic  acid  retained  in  the  blood,  the  normal  respiratory  movements 
cease  and  the  condition  of  asphyxia  ensues,  which  soon  terminates  in  death. 

The  phenomena  of  asphyxia  are  violent  spasmodic  action  of  the  respiratory 
muscles  attended  by  convulsions  of  the  muscles  of  the  extremities,  engorge- 
ment of  the  venous  system,  lividity  of  the  skin,  abolition  of  sensibility  and 
reflex  action,  and  death. 

The  cause  of  death  is  a  paralysis  of  the  heart  from  overdistention  by  blood. 
The  passage  of  the  blood  through  the  capillaries  is  prevented  by  contraction 
of  the  smaller  arteries,  from  irritation  of  the  vasomotor  center.  The  heart 
is  enfeebled  by  a  want  of  oxygen  and  inhibited  in  its  action  by  the  inhibitory 
centers. 


ANIMAL   HEAT.  135 

ANIMAL  HEAT. 

The  functional  activity  of  all  the  organs  and  tissues  of  the  body  is 
attended  by  the  evolution  of  heat,  which  is  independent,  for  the  most  part, 
of  external  conditions.  Heat  is  a  necessary  condition  for  the  due  performance 
of  all  vital  actions;  although  the  body  constantly  loses  heat  by  radiation  and 
evaporation,  it  possesses  the  capability  of  renewing  it  and  of  maintaining  it  at 
a  fixed  standard.  The  normal  temperature  of  the  body  in  the  adult,  as  shown 
by  means  of  a  delicate  thermometer  placed  in  the  axilla,  ranges  from  97.2 50 
F.  to  99. 5°  F.,  though  the  mean  normal  temperature  is  estimated  by  Wunder- 
lich  at  98.60  F. 

The  temperature  varies  in  different  portions  of  the  body  according  to 
the  extent  to  which  oxidation  takes  place,  being  highest  in  the  muscles,  in 
the  brain,  blood,  liver,  etc. 

The  conditions  which  produce  variations  in  the  normal  temperature 
of  the  body  are:  age,  period  of  the  day,  exercise,  food  and  drink,  climate, 
season,  and  disease. 

Age. — At  birth  the  temperature  of  the  infant  is  about  i°  F.  above  that 
of  the  adult,  but  in  a  few  hours  falls  to  95. 50  F.,  to  be  followed  in  the  course 
of  twenty-four  hours  by  a  rise  to  the  normal  or  a  degree  beyond.  During 
childhood  the  temperature  approaches  that  of  the  adult;  in  aged  persons 
the  temperature  remains  about  the  same,  though  they  are  not  so  capable  of 
resisting  the  depressing  effects  of  external  cold  as  adults.  A  diurnal  variation 
of  the  temperature  occurs  from  1 .8°  F.  to  3.70  F.  (Jiirgensen);  the  maximum 
occurring  late  in  the  afternoon,  from  4  to  9  p.  m.  ;  the  minimum,  early  in  the 
morning,  from  1  to  7  A.  M. 

Exercise. — The  temperature  is  raised  from  i°  to  2°  F.  during  active  con- 
tractions of  the  muscular  masses,  and  is  probably  due  to  the  increased 
activity  of  chemic  changes;  a  rise  beyond  this  point  being  prevented  by  its 
diffusion  to  the  surface,  consequent  on  a  more  rapid  circulation,  radiation, 
more  rapid  breathing,  etc. 

Food  and  Drink. — The  ingestion  of  a  hearty  meal  increases  the  tempera- 
ture but  slightly;  an  absence  of  food,  as  in  starvation,  produces  a  marked 
decrease.  Alcoholic  drinks,  in  large  amounts,  in  persons  unaccustomed  to 
their  use,  cause  a  depression  of  the  temperature  amounting  to  from  i°  to  2° 
F.     Tea  causes  a  slight  elevation. 

External  Temperature. — Long-continued  exposure  to  cold,  especially  if 
the  body  is  at  rest,  diminishes  the  temperature  from  i°  to  20  F.,  while  exposure 
to  a  great  heat  slightly  increases  it. 

Disease  frequently  causes  a  marked  variation  in  the  normal  temperature 
of  the  body,  which  rises  as  high  as  1070  F.  in  typhoid  fever  and  1050  F.  in 


136  HUMAN  PHYSIOLOGY. 

pneumonia;  in  cholera  it  falls  as  low  as  8o°  F.  Death  usually  occurs  when 
the  heat  remains  high  and  persistent,  from  1060  to  no°  F.;  the  increase  of 
heat  in  disease  is  due  to  excessive  production-  rather  than  to  diminished 
elimination. 

The  source  of  heat  is  to  be  sought  for  in  the  chemic  decompositions 
and  hydrations  taking  place  during  the  general  process  of  nutrition,  and 
in  the  combustion  of  the  food  materials  by  the  oxygen  of  the  inspired  air; 
the  amount  of  its  production  is  in  proportion  to  the  activity  of  the  internal 
changes. 

Every  contraction  of  a  muscle,  every  act  of  secretion,  each  exhibition  of 
nerve  force,  is  accompanied  by  a  change  in  the  chemic  composition  of  the 
tissues  and  an  evolution  of  heat.  The  reduction  of  the  disintegrated  tissues 
to  their  simplest  form  by  oxidation,  and  the  combination  of  the  oxygen  of  the 
inspired  air  with  the  carbon  and  hydrogen  of  the  blood  and  tissues,  results 
in  the  formation  of  carbonic  acid  and  water  and  the  liberation  of  a  great 
amount  of  heat. 

Certain  elements  of  the  food,  particularly  the  carbo-hydrates  and  the  fats, 
undergo  oxidation  without  taking  part  in  the  formation  of  the  tissues,  being 
transformed  into  carbon  dioxid  and  water,  and  thus  increase  the  sum  of 
heat  in  the  body. 

Heat-producing  Tissues. — All  the  tissues  of  the  body  add  to  the  general 
amount  of  heat,  according  to  the  degree  of  their  activity.  But  special  struc- 
tures, on  account  of  their  mass  and  the  large  amount  of  blood  they  receive, 
are  particularly  to  be  regarded  as  heat  producers,  e.  g. : 

1.  During  mental  activity  the  brain  receives  nearly  one-fifth  of  the  entire 
volume  of  blood,  and  the  venous  blood  returning  from  it  is  charged  with 
waste  matters,  and  its  temperature  is  increased. 

2.  The  muscular  tissue,  on  account  of  the  many  chemic  changes  occurring 
during  active  contractions,  must  be  regarded  as  the  chief  heat-producing 
tissue. 

3.  The  secreting  glands,  during  their  functional  activity,  add  largely  to  the 
amount  of  heat. 

The  entire  quantity  of  heat  generated  within  the  body  has  been  demon- 
strated experimentally  to  be  about  2,300  calories,  a  calory,  or  heat  unit, 
being  that  amount  of  heat  required  to  raise  the  temperature  of  one  kilogram 
of  water  (2.2  pounds)  i°  C.  This  quantity  of  heat,  if  not  utilized  and  retained 
within  the  body,  would  elevate  its  temperature  in  twenty-four  hours  about 
6o°  F.  That  this  volume  of  heat  depends  very  largely  upon  the  oxidation 
of  the  food-stuffs  can  be  shown  experimentally. 


SECRETION.  137 

The  normal  temperature  of  the  body  is  maintained  by  a  constant  expendi- 
ture of  the  heat  in  several  directions: 

1.  In  warming  the  food,  drink,  and  air  that  are  consumed  in  twenty-four 
hours.     For  this  purpose  about  157  heat  units  are  required. 

2.  In  evaporating  water  from  the  skin  and  lungs,  619  heat  units  being  utilized 
for  this  purpose. 

3.  In  radiation  and  conduction.  By  these  processes  the  body  loses  at  least 
fifty  per  cent,  of  its  heat,  or  1,156  heat  units. 

4.  In  the  production  of  work;  the  work  of  the  circulatory,  respiratory,  mus- 
cular, and  nervous  apparatus  being  performed  by  the  transformation  of 
369  heat  units  into  units  of  work. 


SECRETION. 

The  process  of  secretion  consists  in  the  separation  of  materials  from 
the  blood  which  are  either  to  be  again  utilized  to  fulfil  some  special  purpose 
in  the  economy,  or  are  to  be  removed  from  the  body  as  excrementitious 
matter;  in  the  former  case  they  constitute  the  secretions,  in  the  latter,  the 
excretions. 

The  materials  which  enter  into  the  composition  of  the  secreticms  are  derived 
from  the  nutritive  principles  of  the  blood,  and  require  special  organs — e.  g., 
gastric  glands,  mammary  glands,  etc. — for  their  proper  elaboration. 

The  materials  which  compose  the  excretions  preexist  in  the  blood,  and  are 
the  results  of  the  activities  of  the  nutritive  process;  if  retained  within  the  body, 
they  exert  a  deleterious  influence  upon  the  composition  of  the  blood. 

Destruction  of  a  secreting  gland  abolishes  the  secretion  peculiar  to  it,  and 
it  can  not  be  formed  by  any  other  gland;  but  among  the  excreting  organs 
there  exists  a  complementary  relation,  so  that  if  the  function  of  one  organ 
be  interfered  with,  another  performs  it  to  a  certain  extent. 


Classification  of  the  Secretions. 

PERMANENT   FLUIDS. 

Serous  fluids.  Vitreous  humor  of  the  eye. 

Synovial  fluid.  Fluid  of  the  labyrinth  of  the  internal 

Aqueous  humor  of  the  eye.  ear. 

Cerebro-spinal  fluid. 


138  HUMAN  PHYSIOLOGY. 

TRANSITORY   FLUIDS. 

Mucus.  Gastric  juice. 

Sebaceous  matter.  Pancreatic  juice. 

Cerumen  (external  meatus).  Secretion  from  Brunner's  glands. 

Meibomian  fluid.  Secretion  from  Lieberkuhn's  glands. 

Milk  and  colostrum.  Secretions  from  follicles  of  the  large 

Tears.  intestine. 

Saliva.  Bile  (also  an  excretion). 

EXCRETIONS. 

Perspiration  and  the  secretion  of  the     Urine. 

axillary  glands.  Bile  (also  a  secretion). 

FLUIDS   CONTAINING   FORMED   ANATOMIC  ELEMENTS. 

Seminal    fluid,    containing  spermat-    Fluid  of    the  Graafian  follicles  con- 
ozoids.  taining  the  ovum. 

The  essential  apparatus  for  secretion  is  a  delicate,  homogeneous,  struc- 
tureless membrane,  on  one  side  of  which,  in  close  contact,  is  a  capillary  plexus 
of  blood-vessels,  and  on  the  other  side  a  layer  of  cells  the  physiologic  function 
of  which  varies  in  different  situations. 

Secreting  organs  may  be  divided  into  membranes  and  glands. 

Serous  membranes  usually  exist  as  closed  sacs,  the  inner  surfaces  of  which 
are  covered  by  pale,  nucleated  epithelium,  containing  a  small  amount  of 
secretion. 

The  serous  membranes  are  the  pleura,  peritoneum,  pericardium,  synovial 
sacs,  etc. 

The  serous  fluids  are  of  a  pale  amber  color,  somewhat  viscid,  alkaline, 
coagulable  by  heat,  and  resemble  the  serum  of  the  blood;  their  amount  is 
but  small.  The  pleural  fluid  varies  from  four  to  seven  drams;  the  peritoneal 
from  one  to  four  ounces;  the  pericardial  from  one  to  three  drams. 

The  synovial  fluid  is  colorless,  alkaline,  and  extremely  viscid,  from  the 
presence  of  synovin. 

The  function  of  serous  fluids  is  to  moisten  the  opposing  surfaces,  so  as  to 
prevent  friction  during  the  play  of  the  viscera. 

The  mucous  membranes  are  soft  and  velvety  in  character,  and  line  the 
cavities  and  passages  leading  to  the  exterior  of  the  body — e.  g.,  the  gastro- 
intestinal, pulmonary  and  genito-urinary.  They  consist  of  a  primary  base- 
ment membrane  covered  with  epithelial  cells,  which  in  some  situations  are 
tessellated,  in  others,  columnar. 


SECRETION.  139 

Mucus  is  a  pale,  semitransparent,  alkaline  fluid,  containing  epithelial  cells 
and  leukocytes.  It  is  composed,  chemically,  of  water,  an  albuminous  prin- 
ciple (mucin),  and  mineral  salts;  the  principal  varieties  are  nasal,  bronchial, 
vaginal,  and  urinary. 

Secreting  glands  are  formed  of  the  same  elements  as  the  secreting  mem- 
branes, but  instead  of  presenting  flat  surfaces,  are  involuted,  forming  tubules, 
which  may  be  simple  follicles — e.  g.,  mucous,  uterine,  or  intestinal;  or  com- 
pound follicles — e.  g.,  gastric  glands,  mammary  glands,  or  racemose  glands — 
e.  g.,  salivary  glands  and  pancreas.  They  are  composed  of  a  basement 
membrane,  enveloped  by  a  plexus  of  blood-vessels,  and  are  lined  by  epithelial 
and  true  secreting  cells,  which  in  different  glands  possess  the  capability  of 
elaborating  elements  characteristic  of  their  secretions. 

In  the  production  of  the  secretion  two  essentially  different  processes 
are  concerned: 

1.  Chemic. — The  formation  and  elaboration  of  the  characteristic  organic 
ingredients  of  the  secreted  fluids — e.  g.,  pepsin,  pancreatin — take  place 
during  the  intervals  of  glandular  activity,  as  a  part  of  the  general  function 
of  nutrition.  They  are  formed  by  the  cells  lining  the  glands,  and  can  often 
be  seen  in  their  interior  with  the  aid  of  the  microscope — e.  g.,  bile  in  the 
liver  cells,  fat  in  the  cells  of  the  mammary  gland. 

2.  Physical. — Consisting  of  a  transudation  of  water  and  mineral  salts  from 
the  blood  into  the  interior  of  the  gland. 

During  the  intervals  of  glandular  activity  only  that  amount  of  blood 
passes  through  the  gland  sufficient  for  proper  nutrition;  when  the  gland  begins 
to  secrete,  under  the  influence  of  an  appropriate  stimulus,  the  blood-vessels 
dilate  and  the  quantity  of  blood  becomes  greatly  increased  beyond  that 
flowing  to  the  gland  during  its  repose. 

Under  these  conditions  a  transudation  of  water  and  salt  takes  place, 
washing  out  the  characteristic  ingredients,  which  are  discharged  by  the  gland 
ducts.  The  discharge  of  the  secretion  is  intermittent;  they  are  retained  in 
the  glands  until  they  receive  the  appropriate  stimulus,  when  they  pass  into 
the  larger  ducts  by  the  vis  a  tergo,  and  are  then  discharged  by  the  contraction 
of  the  muscular  walls  of  the  ducts. 

The  activity  of  glandular  secretion  is  hastened  by  an  increase  in  the  blood- 
pressure  and  retarded  by  a  diminution. 

The  nerve  centers  in  the  medulla  oblongata  influence  secretion: 

1.  By  increasing  or  diminishing  the  amount  of  blood  entering  a  gland. 

2.  By  exerting  a  direct  influence  upon  the  secreting  cells  themselves,  the 
centers  being  excited  by  reflex  stimulation,  mental  emotion,  etc. 


140  HUMAN  PHYSIOLOGY. 

MAMMARY   GLANDS. 

The  mammary  glands,  which  secrete  the  milk,  are  two  more  or  less 
hemispheric  organs,  situated  in  the  human  female  on  the  anterior  surface 
of  the  thorax.  Though  rudimentary  in  childhood,  they  gradually  increase 
in  size  as  the  young  female  approaches  puberty. 

The  gland  presents  at  its  convexity  a  small  prominence  of  skin  (the  nipple) 
which  is  surrounded  by  a  circular  area  of  pigmented  skin  (the  areola).  The 
gland  proper  is  covered  by  a  layer  of  adipose  tissue  anteriorly  and  is  attached 
posteriorly  to  the  pectoral  muscles  by  a  meshwork  of  fibrous  tissue.  During 
utero-gestation  the  mammary  glands  become  larger,  firmer,  and  more  lobu- 
lated;  the  areola  darkens  and  the  veins  become  more  prominent.  At  the 
period  of  lactation  the  gland  is  the  seat  of  active  histologic  and  physiologic 
changes,  correlated  with  the  production  of  milk.  At  the  close  of  lactation 
the  glands  diminish  in  size,  undergo  involution,  and  gradually  return  to 
their  original  non-secreting  condition. 

Structure  of  the  Mammary  Gland. — The  mammary  gland  consists  of 
an  aggregation  of  some  fifteen  or  twenty  lobes,  each  of  which  is  surrounded 
by  a  framework  of  fibrous  tissue.  The  lobe  is  provided  with  an  excretory 
duct,  which,  as  it  approaches  the  base  of  the  nipple,  expands  to  form  a  sinus 
or  reservoir,  beyond  which  it  opens  by  a  narrowed  orifice  on  the  surface  of 
the  nipple.  On  tracing  the  duct  into  a  lobe,  it  is  found  to  divide  and  sub- 
divide, and  finally  to  terminate  in  lobules  or  acini.  Each  acinus  consists  of  a 
basement  membrane,  lined  by  low  polyhedral  cells.  Externally  it  is  sur- 
rounded by  connective  tissue  supporting  blood-vessels,  lymphatics  and  nerves. 


MILK. 

Milk  is  an  opaque,  bluish-white  fluid,  almost  inodorous,  of  a  sweet  taste, 
an  alkaline  reaction,  and  a  specific  gravity  of  1025  to  1040.  When  examined 
microscopically  it  is  seen  to  consist  of  a  clear  fluid  (the  milk-plasma),  holding 
in  suspension  an  enormous  number  of  small,  highly  refractive  oil-globules, 
which  measure,  on  an  average,  joooo  °f  an  mcn  m  diameter.  Each  globule 
is  supposed  by  some  observers  to  be  surrounded  by  a  thin,  albuminous 
envelope,  which  enables  it  to  maintain  the  discrete  form.  The  quantity  of 
milk  secreted  daily  by  the  human  female  averages  about  two  and  a  half 
pints.  The  milk  of  all  the  mammalia  consists  of  all  the  different  classes  of 
nutritive  principles,  though  in  varying  proportions.  The  relative  proportions 
in  which  these  constituents  exist  are  shown  in  the  following  table  of  analyses: 


MAMMARY    GLANDS. 


141 


COMPOSITION  OF  MILK 


In  100  parts. 

Human. 

Cow. 

Goat. 

Ass. 

Sheep. 

Mare. 

Water 

88.00 

86.87 

87-54 

9x-57 

82  .27 

88.80 

Caseinogen 

2 .40 

3-98 

3.00 

1 .09 

6  .10 

2 .  19 

Lactalbumin .... 

°-57 

0.77 

0.62 

0 .70 

1 .00 

0.42 

Fat 

2  .00 

3-5° 

4.20 

1 .02  ] 

5-3° 

2.50 

Lactose 

5-87 

4.00 

4.00 

5-5°  j 

4.20 

5-5° 

Salts 

0.16 

0.17 

0.56 

0.42 

1 .00 

O  .  ^0 

Caseinogen  is  the  chief  protein  constituent  of  milk,  and  is  held  in  solution 
by  the  presence  of  calcium  phosphate.  On  the  addition  of  acetic  acid  or  of 
sodium  chlorid  up  to  the  point  of  saturation,  the  caseinogen  is  precipitated 
as  such,  and  may  be  collected  by  appropriate  chemic  methods.  When 
taken  into  the  stomach  caseinogen  is  coagulated — that  is,  it  is  separated  into 
casein  or  tyrein  and  a  small  quantity  of  a  new  soluble  protein.  The  ferment 
which  induces  this  change  is  known  as  rennin.  The  presence  of  calcium 
phosphate  is  necessary  for  this  coagulation. 

The  fat  of  milk  is  more  or  less  solid  at  ordinary  temperatures.  It  is  a 
composition  of  olein,  palmitin,  and  stearin,  with  a  small  quantity  of  butyrin 
and  caproin.  When  milk  is  allowed  to  stand  for  some  time  the  fat-globules 
rise  to  the  surface  and  form  a  thick  layer,  known  as  cream.  When  subjected 
to  the  churning  process,  the  fat  globules  run  together  and  form  a  cohesive 
mass — the  butter. 

Lactose  is  the  particular  form  of  sugar  characteristic  of  milk.  It  belongs 
to  the  saccharose  group  and  has  the  following  composition:  C12H22On. 
In  the  presence  of  the  bacilus  acidi  lactici  the  lactose  is  decomposed  into 
lactic  acid  and  carbon  dioxide,  the  former  of  which  will  cause  a  coagulation  of 
the  caseinogen. 

Mechanism  of  Secretion. — During  the  time  of  lactation  the  mammary 
gland  exhibits  periods  of  secretory  activity  which  alternate  with  periods  of 


142  HUMAN  PHYSIOLOGY. 

rest.  Coincidentally  with  these  periods,  certain  histologic  changes  take 
place  in  the  secreting  structures  of  the  gland.  At  the  close  of  a  period  of 
active  secretion  each  acinus  presents  the  following  features:  the  epithelial 
cells  are  short,,  cubic,  nucleated,  and  border  a  relatively  wide  lumen  in  which 
is  to  be  found  a  variable  quantity  of  non-discharged  milk.  After  the  gland 
has  rested  for  some  time,  active  metabolism  again  begins.  The  epithelial 
cells  grow  and  elongate;  the  nucleus  divides  into  two  or  three  new  nuclei,  and 
at  the  same  time  the  cell  becomes  constricted;  the  inner  portion  is  detached 
and  is  discharged  into  the  lumen.  Coincidentally  with  these  changes  oil- 
globules  make  their  appearance  in  the  cell  protoplasm,  some  of  which  are 
discharged  separately  into  the  lumen,  while  others  remain  for  a  time  asso- 
ciated with  the  detached  cell.  From  these  histologic  changes  it  would  ap- 
pear that  the  caseinogen  and  the  fat-globules  are  metabolic  products  of  the 
cell  protoplasm,  and  not  derived  directly  from  the  blood.  That  lactose  has 
a  similar  origin  appears  certain  from  the  fact  that  it  is  formed  independently 
of  carbohydrate  food.  The  water  and  inorganic  salts  are  doubtless  secreted 
by  a  mechanism  similar  to  that  of  all  other  secreting  glands. 

VASCULAR  OR  DUCTLESS  GLANDS. 
INTERNAL  SECRETIONS. 

The  metabolism  of  the  body  generally,  as  well  as  that  of  individual  organs, 
has  been  shown  to  be  related  not  only  to  the  physiologic  activity  of  such 
organs  as  the  liver  and  pancreas,  but  also  to  the  activity  of  the  so-called 
vascular  or  ductless  glands.  The  influence  of  the  pancreas  in  regulating  the 
oxidation  of  sugar,  and  the  influence  of  the  liver  in  the  maintenance  of  the 
general  metabolism  through  the  production  of  glycogen  and  the  formation 
or  urea,  are  now  established  facts.  That  the  vascular  or  ductless  glands 
to  an  equal  extent,  though  perhaps  in  a  different  way,  assist  in  the  main- 
tenance of  physiologic  processes,  appears  certain  from  the  results  of  animal 
experimentation.  The  explanation  given,  and  generally  accepted  at  the 
present  time,  for  the  influence  of  these  glands  is  that  they  produce  specific 
substances,  which  are  poured  into  the  blood  or  lymph  and  carried  direct  to 
the  tissues,  to  the  activities  of  which  they  appear  to  be  essential;  for  without 
these  substances  the  nutrition  of  the  tissues  declines  and  in  a  short  time  a 
fatal  termination  ensues. 

Inasmuch  as  these  partly  unknown  substances  are  formed  by  cell  activity 
and  are  poured  into  the  interstices  of  the  tissues,  they  have  been  termed 
"  internal  secretions."  Though  the  term  internal  secretions  is  applicable 
to  all  substances  which  arise  in  consequence  of  tissue  metabolism,  and  which 


VASCULAR    OR   DUCTLESS   GLANDS.  1 43 

after  being  poured  into  the  blood,  influence  in  varying  degrees  and  ways 
physiological  processes,  yet  the  term  in  this  connection  will  be  applied  only 
to  the  secretions  of  the  thyroid  gland,  hypophysis  cerebri,  and  adrenal 
bodies. 

Thyroid  Gland. — The  thyroid  gland  or  body  consists  of  two  lobes  situated 
on  the  lateral  aspect  of  the  upper  part  of  the  trachea.  Each  lobe  is  pyriform 
in  shape,  the  base  being  directed  downward  and  on  a  level  with  the  fifth  or 
sixth  tracheal  ring.  The  lobe  is  about  50  mm.  in  length,  20  mm.  in  breadth, 
and  25  mm.  in  thickness.  As  a  rule,  the  lobes  are  united  by  a  narrow  band 
or  isthmus  of  the  same  tissue.  In  color  the  gland  is  reddish,  and  it  is  abund- 
antly supplied  with  blood-vessels  and  lymphatics. 

Microscopic  examination  shows  that  the  thyroid  consists  of  an  enormous 
number  of  closed  sacs  or  vesicles,  variable  in  size,  the  largest  not  measuring 
more  than  0.1  mm.  in  diameter.  Each  sac  is  composed  of  a  thin  homo- 
geneous membrane  fined  by  cuboid  epithelium.  The  interior  of  the  sac  in 
adult  life  contains  a  transparent  viscid  fluid,  containing  albumin  and  termed 
"colloid"  substance.  Externally,  the  sacs  are  surrounded  by  a  plexus  of 
capillary  blood-vessels  and  lymphatics.  The  individual  sacs  are  united  and 
supported  by  connective  tissue,  which  forms,  in  addition,  a  covering  for  the 
entire  gland. 

The  Effects  of  the  Removal  of  the  Thyroid. — The  knowledge  at  present 
possessed  as  to  the  function  of  the  thyroid  gland,  especially  in  mammals,  is 
the  outcome  of  a  study  of  the  effects  which  follow  its  arrest  of  development 
in  the  child,  its  degeneration  in  the  adult,  its  extirpation  in  the  human  being 
as  well  as  in  animals.  The  results,  however,  which  follow  its  extirpation 
are  not  always  uniform  in  all  animals;  sufficient  reasons  for  which  lack  of 
uniformity  cannot  always  be  assigned. 

Cretinism,  a  condition  characterized  by  a  want  of  physical  and  mental 
development,  is  associated  with,  if  not  directly  dependent  on,  a  congenital 
absence  or  an  arrested  development  of  the  thyroid,  either  at  the  time  of  birth 
or  during  the  early  years  of  childhood. 

Myxedema,  a  condition  of  the  skin  in  which  there  is  a  hyperplasia  of  the 
connective  tissue,  of  an  embryonic  type,  rich  in  mucin,  is  generally  regarded 
as  one  of  the  effects  of  degenerative  processes  in  the  thyroid.  Partly  in 
consequence  of  this  change  in  the  skin  the  face  becomes  broader,  swollen,  and 
flattened,  giving  rise  to  a  loss  of  expression.  At  the  same  time  the  mind 
becomes  dull,  clouded,  even  approximating  the  idiotic  type.  This  supposed 
infiltration  of  the  skin  with  mucin  was  termed  myxedema  by  Ord,  who  at  the 
same  time  associated  it  with  a  change  in  the  structure  of  the  thyroid  as  a 
result  of  which  it  became  functionally  useless. 


144  HUMAN  PHYSIOLOGY. 

Extirpation  of  the  thyroid,  for  relief  from  symptoms  due  to  grave  patho- 
logic changes,  has  been  followed  in  human  beings  by  symptoms  similar  to 
those  of  myxedema.  To  this  condition  the  terms  operative  myxedema  and 
cachexia  strumipriva  have  been  applied. 

After  the  publication  of  the  history  of  the  myxedema  which  followed  surgical 
removal  of  the  thyroid,  Schiff,  in  1887,  repeated  his  earlier  experiments  on 
dogs,  and  found  again  that  removal  of  the  thyroid  was  speedily  followed  by 
tremors,  convulsions,  and  death.  Similar  experiments  were  made  by  Hors- 
ley  on  monkeys,  with  results  which  resembled  those  characteristic  of  myx- 
edema. Among  the  symptoms  which  developed  within  a  few  days  after  the 
removal  of  the  gland  may  be  mentioned  loss  of  appetite;  fibrillar  contractions 
of  muscles;  tremors;  spasms;  mucinoid  degeneration  of  the  skin,  giving  rise 
to  puffiness  of  the  eyelids  and  face  and  to  a  swollen  condition  of  the  abdomen ; 
hebetude  of  mind,  frequently  terminating  in  idiocy;  fall  of  blood-pressure; 
dyspnea;  albuminuria;  atrophy  of  the  tissues,  followed  by  death  of  the 
animal  in  the  course  of  from  five  to  eight  weeks.  The  complexus  of  symp- 
toms observed  in  monkeys  was  divided  by  Horsley  into  three  stages:  viz., 
the  neurotic,  the  mucinoid,  and  the  atrophic.  It  is  evident  that  the  presence 
of  the  thyroid  is  essential  to  the  normal  activity  of  the  tissues  generally.  As 
to  the  manner  in  which  it  exerts  its  favorable  influence,  there  is  some  differ 
ence  of  opinion.  The  view  that  the  gland  removes  from  the  blood  certain 
toxic  bodies,  rendering  them  innocuous,  and  thus  preserving  the  body  from 
a  species  of  auto-intoxication,  is  gradually  yielding  to  the  more  probable 
view  that  the  epithelium  is  engaged  in  the  secretion  of  a  specific  material, 
which  finds  its  way  into  the  blood  or  lymph  and  in  some  unknown  way  in- 
fluences favorably  tissue  metabolism.  This  view  of  the  function  of  the 
thyroid  is  supported  by  the  fact  that  successful  grafting  of  a  portion  of  the 
thyroid  beneath  the  skin  or  in  the  abdominal  cavity  will  prevent  the  usual 
symptoms  which  follow  thyroidectomy.  The  same  result  is  obtained  by 
the  intravenous  injection  of  thyroid  juice  or  by  the  administration  of  the 
raw  gland.  It  was  shown  by  Murray  that  myxedematous  patients  could  be 
benefited,  and  even  cured,  by  feeding  them  with  fresh  thyroids  or  even  with 
the  dry  extract. 

The  chemic  features  of  the  material  secreted  and  obtained  from  the 
structures  of  the  thyroid  indicate  that  it  is  a  complex  protein  containing 
iodin,  which,  under  the  influence  of  various  reagents,  undergoes  cleavage, 
giving  rise  to  a  non-protein  residue,  which  carries  with  it  the  iodin  and 
phosphorus.  The  amount  of  iodin  in  the  thyroid  varies  from  0.33  to  1 
milligram  for  each  gram  of  tissue.  To  this  compound  the  term  thyroiodin 
has  been  given.  The  administration  of  this  compound  produces  effects 
similar  to  those  which  follow  the  therapeutic  administration  of  the  fresh 


VASCULAR    OR    DUCTLESS    GLANDS.  145 

thyroid  itself;  viz.,  a  diminution  of  all  myxedematous  symptoms.  In  normal 
states  of  the  body,  thyroiodin  influences  very  actively  the  general  metabolism. 
It  gives  rise  to  a  decomposition  of  fats  and  proteins  and  to  a  decline  in  body- 
weight.  In  large  doses  it  may  produce  toxic  symptoms:  e.  g.,  increased 
cardiac  action,  vertigo,  and  glycosuria. 

The  conclusions  as  to  the  functions  of  the  thyroid  gland  which  have 
been  drawn  from  the  results  that  have  followed  its  removal  from  animals 
bv  surgical  procedures,  have  been  made  questionable,  since  the  discovery 
of  the  parathyroid  glands  and  a  study  of  the  phenomena  which  follow  when 
thev  alone  are  removed.  From  their  situation  and  close  relationship  to  the 
thyroid  gland  it  is  generally  accepted,  that  in  the  earlier  experiments,  espe- 
cially those  made  on  cats  and  dogs,  and  some  other  carnivorous  animals,  both 
sets  of  glands  were  removed  and  hence  some  of  the  symptoms  which  developed 
after  the  removal  of  the  thyroids  were  due  to  the  loss  of  function  not  of  the 
thyroid  but  of  the  parathyroids. 

The  fibrillar  contractions,  the  tremors  and  spasms  are  due  to  parathyroid 
removal;  the  myxedema  and  the  failure  of  the  mental  powers,  to  the  re- 
moval of  the  thyroid. 

The  Parathyroids. — The  parathyroids  are  small  bodies,  usually  four 
in  number,  two  on  each  side.  They  are  divided  into  superior  and  inferior. 
The  superior  are  situated  internally  and  on  the  posterior  surface  in  close 
relation  to,  and  frequently  imbedded  in,  the  substance  of  the  thyroid;  the 
inferior  are  situated  externally,  sometimes  in  contact  with,  and  at  other  times 
removed  a  variable  distance  from  the  thyroid.  Microscopically  the  para- 
thyroids consist  of  thick  cords  of  epithelial  cells  separated  by  septa  of  fine 
connective  tissue  and  surrounded  by  capillary  blood-vessels.  Chemic 
analysis  shows  that  they  also  contain  iodin  in  combination  with  some  organic 
compound. 

Effects  of  Parathyroid  Removal. — The  surgical  removal  of  the  para- 
thyroids is  followed  in  the  course  of  from  two  to  five  days  by  the  death  of  the 
animal  preceded  in  most  instances  by  a  series  of  symptoms  which  are  em- 
braced under  the  general  term  "tetany."  These  symptoms  are  fibrillary 
contractions  of  muscles,  tremors,  spasmodic  contractions  and  paralyses 
of  groups  of  muscles  and  not  infrequently  convulsive  seizures  and  coma. 
During  the  convulsion  there  is  an  acceleration  of  the  heart-beat,  and  increase 
in  the  respiratory  movements  which  frequently  become  dyspneic  in  character. 
There  is  also  a  loss  of  appetite,  nausea,  mucous  vomiting,  and  diarrhea. 
Death  may  occur  during  a  convulsion  or  from  coma.     (Morat  and  Doyon.) 

These  results  for  the  most  part  occur  only  when  ail  the  parathyroids 
are  removed.  It  is  asserted  that  even  if  one  gland  is  retained  the  animal 
10 


146  HUMAN  PHYSIOLOGY. 

does  not  die.     The  above  described  symptoms  may  manifest  themselves, 
however,  but  they  are  slight  in  degree. 

The  Hypophysis  Cerebri. — This  is  a  small  body  lodged  in  the  sella 
turcica  of  the  sphenoid  bone.  It  consists  of  an  anterior  lobe,  somewhat  red 
in  color,  and  a  posterior  lobe,  yellowish-gray  in  color.  The  former  is  much 
the  larger  and  partly  embraces  the  latter.  The  anterior  lobe  is  developed 
from  an  invagination  of  the  epiblast  of  the  mouth  cavity,  and  consists  of 
distinct  gland  tissue.  The  posterior  lobe  is  an  outgrowth  from  the  brain, 
and  is  connected  with  the  infundibulum  by  a  short  stalk.  It  has  been 
suggested  that  the  term  infundibular  body  be  reserved  for  the  posterior  lobe. 
This  distinction  appears  to  be  desirable,  inasmuch  as  in  their  origin  and 
structure  they  are  separate  and  distinct  bodies. 

Removal  of  the  hypophysis  cerebri,  or  the  pituitary  body,  is  always  fol- 
lowed by  a  fatal  result,  preceded  by  symptoms  not  unlike  those  which 
follow  removal  of  the  thyroid:  viz.,  anorexia,  tremors,  spasms,  etc.  De- 
generation of  the  hypophysis  has  been  found  in  connection  with  a  hyper- 
trophic condition  of  the  bones  of  the  face  and  extremities,  to  which  the  term 
acromegalia  has  been  given. 

Intravenous  injection  of  an  extract  of  the  hypophysis  increases  the  force 
of  the  heart-beat  without  any  change  in  its  frequency,  and  causes  a  rise 
of  blood-pressure  from  a  stimulation  of  the  arterioles  (Schaf er  and  Oliver) . 
The  material  secreted  by  the  hypophysis  has  not  been  isolated,  hence  its 
chemic  features  are  unknown.  After  its  formation  it  probably  passes  through 
a  system  of  ducts  into  the  cerebrospinal  fluid,  after  which  it  influences  the 
metabolism  of  the  nervous  and  osseous  tissues  as  well  as  the  force  of  the  heart 
muscle. 

An  extract  of  the  hypophysis  itself  exerts  no  appreciable  effect  on  the  blood- 
pressure  or  on  the  rate  of  the  heart-beat,  nor  does  it  influence  the  circulator)' 
and  respiratory  organs  (Howell).  An  extract  of  the  infundibular  body 
intravenously  injected,  however,  gives  rise  to  increased  blood-pressure  and 
to  a  slowing  of  the  heart-beat. 

Adrenal  Bodies,  or  Suprarenal  Capsules. — These  are  two  flattened 
bodies,  somewhat  crescentic  or  triangular  in  shape,  situated  each  upon  the 
upper  extremity  of  the  corresponding  kidney,  and  held  in  place  by  con- 
nective tissue.  They  measure  about  40  mm.  in  height,  30  mm.  in  breadth, 
and  from  6  to  8  mm.  in  thickness.     The  weight  of  each  is  about  4  gm. 

Function  of  the  Adrenal  Bodies. — It  was  observed  by  Addison  that 
a  profound  disturbance  of  the  nutrition,  characterized  by  a  bronze-like 
discolorization  of  the  skin  and  mucous  membranes  of  the  mouth,  extreme 


VASCULAR    OR   DUCTLESS    GLANDS.  147 

muscular  weakness,  and  profound  anemia,  was  associated  with,  if  not  de- 
pendent on,  pathologic  conditions  of  the  suprarenal  capsules.  In  the  progress 
of  the  disease  the  asthenia  gradually  increases,  the  heart  becomes  weak, 
the  pulse  small,  soft,  and  feeble,  indicating  a  general  loss  of  tone  of  the  mus- 
cular and  vascular  apparatus.  Death  ensues  from  paralysis  of  the  respira- 
tory muscles.  The  essential  nature  of  the  lesion  which  gives  rise  to  these 
symptoms  has  not  been  determined. 

Removal  of  these  bodies  from  various  animals  is  invariably  and  in  a 
short  time  followed  by  death,  preceded  by  some  of  the  symptoms  character- 
istic of  Addison's  disease.  Their  development,  however,  was  more  acute. 
From  the  fact  that  animals  so  promptly  die  from  extirpation  of  these  bodies, 
and  the  further  fact  that  the  blood  of  such  animals  is  toxic  to  those  the  sub- 
jects of  recent  extirpation,  but  not  to  normal  animals,  the  conclusion  was 
drawn  that  the  function  of  the  adrenal  bodies  was  to  remove  from  the  blood 
some  toxic  material  the  product  of  muscle  metabolism.  Its  accumulation 
after  extirpation  gives  rise  to  death  through  auto-intoxication. 

On  the  supposition  that  the  adrenals  might  secrete  and  pour  into  the  blood 
a  specific  material  which  favorably  influences  general  metabolism,  Schafer 
and  Oliver  injected  hypodermically  glycerin  and  water  extracts,  and  observed 
at  once  an  increased  activity  of  the  heart-beats  and  of  the  respiratory  move- 
ments. The  effects,  however,  were  only  transitory.  When  these  extracts 
are  injected  into  the  veins  directly,  there  follows  in  a  short  time  a  cessation 
of  the  auricular  contraction  of  the  heart,  though  the  ventricular  contraction 
continues  with  an  independent  rhythm.  If  the  vagi  are  cut  previous  to  the 
injection  or  if  the  inhibition  is  removed  by  atropin,  the  rapidity  and  vigor 
of  both  auricles  and  ventricles  are  increased.  Whether  the  inhibitory  in- 
fluence is  removed  or  not,  there  is  a  marked  increase  in  the  blood-pressure, 
though  it  is  greater  in  the  former  than  in  the  latter  instance.  This  is  attri- 
buted to  a  direct  stimulation  and  contraction  of  the  muscle-fibers  of  the  arteri- 
oles themselves,  and  not  to  vaso-motor  influences,  as  it  occurs  also  after 
division  of  the  cord  and  destruction  of  the  bulb.  The  contraction  of  the 
arterioles  is  quite  general,  as  shown  by  plethysmographic  studies  of  the  limbs, 
spleen,  kidney,  etc.  Applied  locally  to  the  mucous  membranes,  the  adrenal 
extract  produces  contraction  of  the  blood-vessels  and  pallor.  The  skeletal 
muscles  are  affected  by  the  extract  very  much  as  they  are  by  veratrin.  The 
duration  of  a  single  contraction  is  very  much  prolonged,  especially  in  the 
phase  of  relaxation  or  of  decreasing  energy. 

It  is  evident  from  these  experiments  that  the  adrenal  bodies  are  engaged 
in  elaborating  and  pouring  into  the  blood  a  specific  material  which  stimulates 
to  increased  activity  the  muscle-fibers  of  the  heart  and  arteries,  and  thus 
assists  in  maintaining  the  normal  blood-pressure  as  well  as  the  tonicity  of 


I48  HUMAN  PHYSIOLOGY. 

the  skeletal  muscles.     The  active  principle  of  this  gland  has  been  isolated 
by  Abel  and  termed  epinephrin  or  adrenalin. 

EXCRETION. 

The  principal  excrementitious  fluids  discharged  from  the  body  are  the 
urine,  perspiration,  and  bile;  they  hold  in  solution  principles  of  waste  which 
are  generated  during  the  activity  of  the  nutritive  process  and  are  the  ultimate 
forms  to  which  the  organic  constituents  are  reduced  in  the  body.  They  also 
contain  inorganic  salts. 

The  urinary  apparatus  consists  of  the  kidneys,  ureters,  and  bladder. 

KIDNEYS. 

The  kidneys  are  the  organs  for  the  secretion  of  urine;  they  resemble  a 
bean  in  shape,  are  from  four  to  five  inches  in  length,  two  in  breadth,  and 
weigh  from  four  to  six  ounces. 

They  are  situated  in  the  lumbar  region,  one  on  each  side  of  the  vertebral 
column  behind  the  peritoneum,  and  extend  from  the  eleventh  rib  to  the  crest 
of  the  ilium;  the  anterior  surface  is  convex,  the  posterior  surface  concave, 
the  latter  presenting  a  deep  notch,  the  hilus. 

The  kidney  is  surrounded  by  thin,  smooth  membrane  composed  of  white 
fibrous  and  yellow  elastic  tissue;  though  it  is  attached  to  the  surface  of  the 
kidney  by  minute  processes  of  connective  tissue,  it  can  be  readily  torn  away. 
The  substance  of  the  kidney  is  dense,  but  friable. 

Upon  making  a  longitudinal  section  of  the  kidney  it  will  be  observed  that 
the  hilus  extends  into  the  interior  of  the  organ  and  expands  to  form  a  cavity 
known  as  the  sinus.  This  cavity  is  occupied  by  the  upper,  dilated  portion 
of  the  ureter,  the  interior  of  which  forms  the  pelvis.  The  ureter  subdivides 
into  several  portions,  which  ultimately  give  origin  to  a  number  of  smaller 
tubes,  termed  calyces,  which  receive  the  apices  of  the  pyramids. 

The  parenchyma  of  the  kidney  consists  of  two  portions — viz.: 

1.  An  internal  or  medullary  portion,  consisting  of  a  series  of  pyramids  or 
cones,  some  twelve  or  fifteen  in  number.  They  present  a  distinctly  striated 
appearance,  a  condition  due  to  the  straight  direction  of  the  tubules  and 
blood-vessels. 

2.  An  external  or  cortical  portion,  consisting  of  a  delicate  matrix  containing 
an  immense  number  of  tubules  having  a  markedly  convoluted  appearance. 
Throughout  its  structure  are  found  numerous  small  ovoid  bodies,  termed 
Malpighian  corpuscles. 


KIDNEYS. 


149 


The  Uriniferous  Tubules. — The  kidney  is  a  compound,  tubular  gland 
composed  of  microscopic  tubules  whose  function  it  is  to  secrete  from  the 
blood  those  waste  products  which  collectively  constitute  the  urine.     If  the 

1" 


2"  1  /I' 


Fig.    19. — Longitudinal  Section   through   the   Kidney,   the   Pelvis   of   the 
Kidney,  and  a  Number  of  Renal  Calyces. — (Tyson,  after  Henle.) 

A.  Branch  of  the  renal  artery.  U.  Ureter.  C.  Renal  calyx.  1.  Cortex.  1'. 
Medullary  rays.  1".  Labyrinth,  or  cortex  proper.  2.  Medulla.  2'.  Papillary 
portion  of  medulla,  or  medulla  proper.  2".  Border  layer  of  the  medulla.  3,  3. 
Transverse  section  through  the  axes  of  the  tubules  of  the  border  layer.  4.  Fat  of 
the  renal  sinus.      5,5.  Arterial  branches.      *Transversely  coursing  medulla  rays. 

apex  of  each  pyramid  be  examined  with  a  lens,  it  will  present  a  number  of 
small  orifices,  which  are  the  beginning  of  the  uriniferous  tubules.  From 
this  point  the  tubules  pass  outward  in  a  straight  but  somewhat  divergent 
manner  toward  the  cortex,  giving  off  at  acute  angles  a  number  of  branches 


i5o 


HUMAN  PHYSIOLOGY. 


(Fig.  20).  From  the  apex  to  the  base  of  the  pyramids  they  are  known  as  the 
tubules  of  Bellini.  In  the  cortical  portion  of  the  kidney  each  tubule  becomes 
enlarged  and  twisted,  and  after  pursuing  an  extremely  convoluted  course, 
turns  backward  into  the  medullary  portion  for  some  distance,  forming  the 
descending  limb  of  Henle's  loop:  it  then  turns  upon  itself,  forming  the  ascend- 
ing limb  of  the  loop,  reenters  the  cortex,  again  expands,  and  finally  terminates 
in  a  spheric  enlargement  known  as  Mailer's  or 
Bowman's  capsule.  •  Within  this  capsule  is  contained 
a  small  tuft  of  blood-vessels,  constituting  the  glomer- 
ulus, or  Malpighian  corpuscles. 

Structure  of  the  Tubules. — Each  tubule  consists 
of  a  basement  membrane  lined  by  epithelium  cells 
throughout  its  entire  extent.  The  tubule  and  its 
contained  epithelium  vary  in  shape  and  size  in 
different  parts  of  its  course.  The  termination  of 
the  convoluted  tube  consists  of  a  little  sac  or  cap- 
sule, which  is  ovoid  in  shape  and  measures  about 
giro  of  an  inch.  This  capsule  is  lined  by  a  layer  of 
flattened  epithelial  cells,  which  is  also  reflected  over 
the  surface  of  the  glomerulus.  During  the  periods 
of  secretory  activity  the  blood-vessels  of  the  glomer- 
ulus become  filled  with  blood,  so  that  the  cavity  of 
the  sac  is  almost  obliterated;  after  secretory  activity 
the  blood-vessels  contract  and  the  sac-cavity  be- 
comes enlarged.  In  that  portion  of  the  tubule 
lying  between  the  capsule  and  Henle's  loop  the 
epithelial  cells  are  cuboid  in  shape;  in  Henle's  loop 
they  are  flattened,  while  in  the  remainder  of  the 
tubule  they  are  cuboid  and  columnar. 


Fig.  20. — Diagram- 
matic Exposition  of 
the  Method  in  which 
the  uriniferous 
Tubes  Unite  to  Form 
Primitive  Cones. — 
{Tyson,  after  Ludwig.) 


Blood-vessels  of  the  Kidney. — The  renal  artery  is  of  large  size  and 
enters  the  organ  at  the  hilum;  it  divides  into  several  large  branches,  which 
penetrate  the  substance  of  the  kidney  between  the  pyramids,  at  the  base  of 
which  they  form  an  anastomosing  plexus,  which  completely  surrounds  them. 
From  this  plexus  vessels  follow  the  straight  tubes  toward  the  apex,  white 
others,  entering  the  cortical  portion,  divide  into  small  twigs,  which  enter 
the  Malpighian  body  and  form  a  mass  of  convoluted  vessels,  the  glomerulus. 
After  circulating  through  the  Malpighian  tuft,  the  blood  is  gathered  together 
by  two  or  three  small  veins,  which  again  subdivide  and  form  a  fine  capillary 
plexus,  which  envelops  the  convoluted  tubules;  from  this  plexus  the  veins 
converge  to  form  the  emulgent  vein,  which  empties  into  the  vena  cava. 


KIDNEYS.  151 

The  nerves  of  the  kidney  follow  the  course  of  the  blood-vessels  and 
are  derived  from  the  renal  plexus. 

The  ureter  is  a  membranous  tube,  situated  behind  the  peritoneum 
about  the  diameter  of  a  goose-quill,  eighteen  inches  in  length,  and  extends 
from  the  pelvis  of  the  kidney  to  the  base  of  the  bladder,  which  it  perforates 
in  an  oblique  direction.  It  is  composed  of  three  coats:  fibrous,  muscle 
and  mucous. 

The  bladder  is  a  reservoir  for  the  temporary  reception  of  the  urine  prior 
to  its  expulsion  from  the  body;  when  fully  distended  it  is  ovoid  in  shape, 
and  holds  about  one  pint.  It  is  composed  of  four  coats;  serous,  muscle 
(the  fibers  of  which  are  arranged  longitudinally  and  circularly),  areolar,  and 
mucous.  The  orifice  of  the  bladder  is  controlled  by  the  sphincter  vesica,  a 
muscular  band  about  \  of  an  inch  in  width. 

As  soon  as  the  urine  is  formed  it  passes  through  the  tubuli  uriniferi 
into  the  pelvis  and  thence  through  the  ureters  into  the  bladder,  which  it 
enters  at  an  irregular  rate.  Shortly  after  a  meal,  after  the  ingestion  of 
large  quantities  of  fluid,  and  after  exercise,  the  urine  flows  into  the  bladder 
quite  rapidly,  while  it  is  reduced  to  a  few  drops  during  the  intervals  of 
digestion.  It  is  prevented  from  regurgitating  into  the  ureters  by  the  oblique 
direction  they  take  between  the  mucous  and  muscular  coats. 

Nerve  Mechanism  of  Urination. — When  the  urine  has  passed  into 
the  bladder,  it  is  there  retained  by  the  sphincter  vesicae  muscle,  kept  in  a 
state  of  chronic  contraction  by  the  action  of  a  nerve  center  in  the  lumbar 
region  of  the  spinal  cord.  This  center  can  be  inhibited  and  the  sphincter 
relaxed,  either  reflexly,  by  impressions  coming  through  sensory  nerves 
from  the  mucous  membrane  of  the  bladder,  or  directly,  by  a  voluntary 
impulse  descending  the  spinal  cord.  When  the  desire  to  urinate  is  expe- 
rienced, impressions  made  upon  the  vesical  sensory  nerves  are  carried  to 
the  centers  governing  the  sphincter  and  detrusor  urince  muscles  and  to  the 
brain.  If  now  the  act  of  urination  is  to  take  place,  a  voluntary  impulse 
originating  in  the  brain  passes  down  the  spinal  cord  and  still  further  inhibits 
the  sphincter  vesicas  center,  with  the  effect  of  relaxing  the  muscle  and  of 
stimulating  the  center  governing  the  detrusor  muscle,  with  the  effect  of 
contracting  the  muscle  and  expelling  the  urine.  If  the  act  is  to  be  suppressed, 
voluntary  impulses  inhibit  the  detrusor  center  and  possibly  stimulate  the 
sphincter  center. 

The  genitospinal  center  controlling  these  movements  is  situated  in  that 
portion  of  the  spinal  cord  corresponding  to  the  origin  of  the  third,  fourth, 
and  fifth  sacral  nerves. 


152 


HUMAN  PHYSIOLOGY. 


URINE. 

Normal  urine  is  of  a  pale  yellow  or  amber  color,  perfectly  transparent, 
with  an  aromatic  odor,  an  acid  reaction,  a  specific  gravity  of  1020,  and  a 
temperature  when  first  discharged  of  ioo°  F. 

The  color  varies  considerably  in  health,  from  a  pale  yellow  to  a  brown 
hue,  owing  to  the  presence  of  the  coloring-matter,  urobilin  or  urochrome. 

The  transparency  is  diminished  by  the  presence  of  mucus,  the  calcium 
and  magnesium  phosphates,  and  the  mixed  urates. 

The  reaction  of  the  urine  is  acid,  owing  to  the  presence  of  acid  phosphate 
of  sodium.  The  degree  of  acidity,  however,  varies  at  different  periods 
of  the  day.  Urine  passed  in  the  morning  is  strongly  acid,  while  that  passed 
during  and  after  digestion,  especially  if  the  food  is  largely  vegetable  in 
character,  is  either  neutral  or  alkaline. 

The  specific  gravity  varies  from  1015  to  1025. 

The  quantity  of  urine  excreted  in  twenty-four  hours  is  between  forty  and 
fifty  fluidounces,  but  ranges  above  and  below  this  standard. 

The  odor  is  characteristic,  and  caused  by  the  presence  of  taurylic  and 
phenylic  acids,  but  is  influenced  by  vegetable  foods  and  other  substances 
eliminated  by  the  kidneys. 


COMPOSITION   OF   URINE. 

Water 

Urea 

Other    nitrogenized    crystalline    bodies,    uric    acid, 

principally  in  the  form  of  alkaline  urates, 
Creatin,  creatinin,  xanthin,  hypoxanthin, 
Hippuric  acid,  leucin,   tyrosin,   taurin,   cystin,  all  in 

small  amounts,  and  not  constant, 
Mucus  and  pigment, 


Salts 


Inorganic:  principally  sodium  and  potassium  sul- 
phates, phosphates,  and  chlorids,  with  magnesium 
and  calcium  phosphates,  traces  of  silicates  and 
chlorids, 

Organic:  lactates,  hippurates,  acetates,  formates, 
which  appear  only  occasionally, 

Sugar 

Gases  (nitrogen  and  carbonic  acid  principally). 


967  .0 
14.230 


10.635 


8.135 


a  trace 


1,000.000 


KIDNEYS.  153 

The  average  quantity  of  the  principal  constituents  excreted  in  twenty- 
four  hours  is  as  follows: 

Water 520    fluidounces. 

Urea 512.4    grains. 

Uric  acid 8.5    grains. 

Phosphoric  acid 45  .0    grains. 

Sulphuric  acid 31.11  grains. 

Inorganic  salts 323  .  25  grains. 

Lime  and  magnesia 6.5    grains. 

To  determine  the  amount  of  solid  matters  in  any  given  amount  of  urine 
multiply  the  last  two  figures  of  the  specific  gravity  by  the  coefficient  of 
Haeser,  2.33 — e.  g.,  in  1,000  grains  of  urine  having  a  specific  gravity  of  1022, 
there  are  contained  22X2.33  =51.26  grains  of  solid  matter. 

Organic  Constituents  of  Urine. — Urea  is  one  of  the  most  important 
of  the  organic  constituents  of  the  urine,  and  is  present  to  the  extent  of  from 
2.5  to  3.2  per  cent.  Urea  is  a  colorless,  neutral  substance,  crystallizing  in 
four-sided  prisms  terminated  by  oblique  surfaces.  When  crystallization  is 
caused  to  take  place  rapidly,  the  crystals  take  the  form  of  long,  silky  needles. 
Urea  is  soluble  in  water  and  alcohol;  when  subjected  to  prolonged  boiling, 
it  is  decomposed,  giving  rise  to  carbonate  of  ammonia.  In  the  alkaline 
fermentation  of  urine,  urea  takes  up  two  molecules  of  water  with  the  pro- 
duction of  carbonate  of  ammonia. 

The  average  amount  of  urea  excreted  daily  has  been  estimated  at  about 
500  grains.  As  urea  is  one  of  the  principal  products  of  the  breaking  up  of  the 
protein  compounds  within  the  body,  it  is  quite  evident  that  the  quantity 
produced  and  eliminated  in  twenty-four  hours  will  be  increased  by  any 
increase  in  the  amount  of  protein  food  consumed,  or  by  a  rapid  destruction 
of  protein  tissues,  as  is  observed  in  various  pathologic  states,  inanition, 
febrile  conditions,  fevers,  etc.  A  farinaceous  or  vegetable  diet  will  diminish 
the  urea  production  nearly  one  half. 

Muscular  exercise  when  the  nutrition  of  the  body  is  in  a  state  of  equilibrium 
does  not  seem  to  increase  the  quantity  of  urea. 

Seat  of  Urea  Formation. — As  to  the  seat  of  urea  formation,  little  is  posi- 
tively known.  It  is  quite  certain  that  it  preexists  in  the  blood  and  is  merely 
excreted  by  the  kidneys.  It  is  not  produced  in  muscles,  as  even  after  pro- 
longed exercise  hardly  a  trace  of  urea  is  to  be  found  in  them.  Experimental 
and  pathologic  facts  point  to  the  liver  as  the  probable  organ  engaged  in  urea 
formation.  Acute  yellow  atrophy  of  the  liver,  suppurative  diseases  of  the 
liver,  diminish  almost  entirely  the  production  of    urea.     It  is  at  present 


154  HUMAN  PHYSIOLOGY. 

believed  that  urea  is  derived  from  ammonium  compounds  and  from  amino- 
acids  absorbed  from  the  intestine. 

Uric  acid  is  also  a  constant  ingredient  of  the  urine  and  is  closely  allied  to 
urea.  It  is  a  nitrogen-holding  compound,  carrying  out  of  the  body  a  portion 
of  the  nitrogen.  The  amount  eliminated  daily  varies  from  five  to  ten  grains. 
Uric  acid  is  a  colorless  crystal  belonging  to  the  rhombic  system.  It  is  in- 
soluble in  water,  and  if  eliminated  in  excessive  amounts,  it  is  deposited  as  a 
"brick-red"  sediment  in  the  urine.  It  is  doubtful  if  uric  acid  exists  in  a 
free  state,  being  combined  for  the  most  part  with  sodium  and  potassium 
bases  forming  urates.  It  is  to  be  regarded  as  one  of  the  terminal  products 
of  the  decomposition  of  nucleic  acid  which  in  turn  is  derived  from  nuclein, 
a  constituent  of  cell  nuclei. 

Hippuric  acid  is  found  very  generally  in  urine,  though  it  is  present  only 
in  small  amounts.  It  is  increased  by  a  diet  as  asparagus,  cranberries,  plums, 
and  by  the  administration  of  benzoic  and  cinnamic  acids.  It  is  probably 
formed  in  the  kidney. 

Kreatinin  resembles  the  kreatin  derived  from  muscles.  It  is  a  colorless 
crystal,  belonging  to  the  rhombic  system.  Its  origin  is  unknown,  though  it 
is  largely  increased  in  amount  by  albuminous  food.  About  fifteen  grains 
are  excreted  daily. 

Xanthin,  hypo-xanthin,  and  guanin  are  also  constituents  of  urine. 
They  are  nitrogenized  compounds  and  are  also  terminal  products  of  nucleic 
acid. 

Urobilin,  the  coloring-matter  of  the  urine,  is  a  derivative  of  the  bile 
pigments.  It  is  particularly  abundant  in  febrile  conditions,  giving  to  the 
urine  its  reddish-yellow  color. 

Inorganic  Constituents  of  Urine. — Earthy  Phosphate.  Phosphoric 
acid  in  combination  with  magnesium  and  calcium  is  excreted  daily  to  the 
extent  of  from  fifteen  to  thirty  grains.  The  phosphates  are  insoluble  in 
water,  but  are  held  in  solution  in  the  urine  by  its  acid  ingredients,  alkalinity 
of  the  urine  being  attended  with  a  copious  precipitation  of  the  phosphates. 
Mental  work  increases  the  amount  of  phosphoric  acid  excreted,  a  condition 
caused  by  increased  metabolism  of  the  nervous  tissue. 

Sulphuric  acid  in  combination  with  sodium  and  potassium  constitutes 
the  sulphates,  of  which  about  thirty  grains  are  excreted  daily.  Sulphuric 
acid  results  largely  from  the  decomposition  of  albuminous  food  and  from 
increased  destruction  of  animal  tissues. 

The  gases  of  urine  are  carbonic  acid  and  nitrogen. 


LIVE?.  155 

Mechanism  of  Urinary  Secretion. — As  the  kidney  anatomically  presents 
an  apparatus  for  filtration  (the  Malpighian  bodies)  and  an  apparatus  for 
secretion  (the  epithelial  cells  of  the  urinary  tubules),  it  might  be  inferred  that 
the  elimination  of  the  constituents  of  the  urine  is  accomplished  by  the  two- 
fold process  of  filtration  and  secretion;  that  the  water  and  highly  diffusible 
inorganic  salts  simply  pass  by  diffusion  through  the  walls  of  the  blood-vessels 
of  the  glomerulus  into  the  capsule  of  Muller,  while  the  urea  and  remaining 
organic  constituents  are  removed  by  true  secretory  action  of  the  renal  epi- 
thelium.    Modern  experimentation  supports  this  view  of  renal  action. 

The  secretion  of  urine  is,  therefore,  partly  physical  and  partly  vital. 

The  filtration  of  urinary  constituents  from  the  glomerulus  into  Miiller's 
capsule  depends  largely  upon  the  blood-pressure  and  the  rapidity  of  blood 
flow  in  the  renal  artery  and  glomerulus.  Among  the  influences  which  in- 
crease the  pressure  and  velocity  may  be  mentioned  increased  frequency  and 
force  of  the  heart's  action,  contraction  of  the  capillary  vessels  of  the  body 
generally,  dilatation  of  the  renal  artery,  and  increase  in  the  volume  of  the 
blood. 

The  reverse  conditions  lower  the  blood-pressure  and  diminish  the  secretion 
of  urine. 

The  fact  that  organic  matters  are  eliminated  by  the  secretory  activity  of  the 
renal  epithelium  seems  to  be  well  established  by  modern  experiments. 
These  substances,  removed  from  the  blood  in  the  secondary  capillary  plexus 
of  blood-vessels,  by  a  true  selective  action  of  the  epithelium,  are  dissolved 
and  washed  toward  the  pelves  by  the  liquid  coming  from  the  capsules. 

The  blood-supply  to  the  kidney  is  regulated  by  the  nervous  system.  If 
the  renal  nerves  be  divided,  the  renal  artery  dilates  and  a  copious  flow  of 
urine  takes  place.  If  the  peripheral  ends  of  the  same  nerves  be  stimulated. 
the  artery  contracts  and  the  urinary  flow  ceases.  The  same  is  true  of  the 
splanchnic  nerves,  through  which  the  vaso-motor  nerves  coming  from  the 
medulla  oblongata  and  spinal  cord  pass  to  the  renal  plexus. 


LIVER. 

The  liver  is  a  highly  vascular,  conglomerate  gland,  appended  to  the 
alimentary  canal.  It  is  the  largest  gland  in  the  body,  weighing  about  four 
and  one  half  pounds;  it  is  situated  in  the  right  hypochondriac  region,  and  is 
retained  in  position  by  five  ligaments,  four  of  which  are  formed  by  duplica- 
tures  of  the  peritoneal  investment. 

The  proper  coat  of  the  liver  is  a  thin  but  firm  fibrous  membrane,  closely 
adherent  to  the  surface  of  the  organ,  which  it  penetrates  at  the  transverse 


156  HUMAN  PHYSIOLOGY. 

fissure,  and  follows  the  vessels  in  their  ramifications  through  its  substance, 
constituting  Glisson's  capsule. 

Structure  of  the  Liver. — The  liver  is  made  up  of  a  large  number  of  small 
bodies  (the  lobules),  rounded  or  ovoid  in  shape,  measuring  ^  of  an  inch  in 
diameter,  separated  by  a  space  in  which  are  situated  blood-vessels,  nerves, 
hepatic  ducts,  and  lymphatics. 

The  lobules  are  composed  of  cells,  which,  when  examined  microscopic- 
ally, exhibit  a  rounded  or  polygonal  shape,  and  measure,  on  the  average, 
T0V0  °f  an  incn  in  diameter;  they  possess  one,  and  sometimes  two,  nuclei; 
they  also  contain  globules  of  fat,  pigment  matter,  and  animal  starch.  The 
cells  constitute  the  secreting  structure  of  the  liver,  and  are  the  true  hepatic  cells. 

The  blood-vessels  which  enter  the  liver  are: 

1 .  The  portal  vein,  made  up  of  the  gastric,  splenic,  and  superior  and  inferior 
mesenteric  veins. 

2.  The  hepatic  artery,  a  branch  of  the  celiac  axis. 

Both  the  portal  vein  and  the  hepatic  artery  are  invested  by  a  sheath  of 
areolar  tissue. 

The  vessels  which  leave  the  liver  are  the  hepatic  veins,  originating  in  its 
interior,  collecting  the  blood  distributed  by  the  portal  vein  and  hepatic  artery, 
and  conducting  it  to  the  ascending  vena  cava. 

Distribution  of  Vessels. — The  portal  vein  and  the  hepatic  artery,  upon  en- 
tering the  liver,  penetrate  its  substance,  divide  into  smaller  and  smaller 
branches,  occupy  the  spaces  between  the  lobules,  completely  surrounding  and 
limiting  them,  and  constitute  the  interlobular  vessels.  The  hepatic  artery,  in  its 
course,  gives  off  branches  to  the  walls  of  the  portal  vein  and  Glisson's  capsule, 
and  finally  empties  into  the  small  branches  of  the  portal  vein  in  the  interlobular 
spaces. 

The  interlobular  vessels  form  a  rich  plexus  around  the  lobules,  from  which 
branches  pass  to  neighboring  lobules  and  enter  their  substance,  where  they 
form  a  very  fine  network  of  capillary  vessels,  ramifying  over  the  hepatic 
cells,  in  which  the  various  functions  of  the  liver  are  performed.  The  blood 
is  then  collected  by  small  veins,  converging  toward  the  center  of  the  lobule, 
to  form  the  intralobular  vein,  which  runs  through  its  long  axis  and  empties 
into  the  sublobular  vein.  The  hepatic  veins  are  formed  by  the  union  of  the 
sub  lobular  veins,  and  carry  the  blood  to  the  ascending  vena  cava ;  their  walls 
are  thin  and  adherent  to  the  substance  of  the  hepatic  tissue. 

The  hepatic  ducts  or  bile  capillaries  originate  within  the  lobules,  in  a 
very  fine  plexus  lying  between  the  hepatic  cells;  whether  the  smallest  vessels 


LIVER.  157 

have  distinct  membranous  walls,  or  whether  they  originate  in  the  spaces 
between  the  cells  by  open  orifices,  has  not  been  satisfactorily  determined. 

The  bile-channels  empty  into  the  interlobular  ducts,  which  measure  about 
soVo  OI  an  mcn  m  diameter  and  are  composed  of  a  thin,  homogeneous 
membrane  lined  by  flattened  epithelial  cells. 

As  the  interlobular  bile-ducts  unite  to  form  large  trunks,  they  receive  an 
external  coat  of  fibrous  tissue,  which  strengthens  their  walls;  they  finally 
unite  to  form  one  large  duct  (the  hepatic  duct),  which  joins  the  cystic  duct; 
the  union  of  the  two  forms  the  ductus  communis  choledochus,  which  is  about 
three  inches  in  length,  the  size  of  a  gooze-quill,  and  opens  into  the  duodenum. 

The  gall-bladder  is  a  pear-shaped  sac,  about  four  inches  in  length, 
situated  in  a  fossa  on  the  under  surface  of  the  liver.  It  is  a  reservoir  for  the 
bile,  and  is  capable  of  holding  about  one  ounce  and  a  half  of  fluid.  It  is 
composed  of  three  coats: 

1.  Serous,  a  reflection  of  the  peritoneum. 

2.  Fibrous  and  muscular. 

3.  Mucous. 

Functions  of  the  Liver. — The  fiver  is  a  complex  organ  having  a  variety 
of  relations  to  the  general  processes  of  the  body.  While  its  physiologic 
actions  are  not  yet  wholly  understood,  it  may  be  said  that  it  is  engaged: 

1.  In  the  secretion  of  bile. 

2.  In  the  production  of  starch  (glycogen)  and  sugar  (glucose). 

3.  In  the  formation  of  urea. 

The  Secretion  of  Bile. — The  characteristic  constituents  of  the  bile  do  not 
preexist  in  the  blood,  but  are  formed  in  the  interior  of  the  liver  cells  of 
materials  derived  from  the  venous  and  arterial  blood.  The  hepatic  cells, 
absorbing  these  materials,  elaborate  them  into  bile-elements,  and  in  so  doing 
undergo  histologic  changes  similar  to  those  exhibited  by  other  secretory 
glands.  The  bile  once  formed,  it  passes  into  the  months  of  the  bile  capil- 
laries, near  the  periphery  of  the  lobules.  Under  the  influence  of  the  vis 
a  tergo  of  the  new-formed  bile  it  flows  from  the  smaller  into  the  large 
bile-ducts,  and  finally  empties  into  the  intestine,  or  is  regurgitated  into  the 
gall-bladder,  where  it  is  stored  up  until  it  is  required  for  the  digestive  process 
in  the  small  intestine.  The  study  of  the  secretion  of  bile  by  means  of  biliary 
fistuke  reveals  the  fact  that  the  secretion  is  continuous  and  not  intermittent; 
that  the  hepatic  cells  are  constantly  pouring  bile  into  the  ducts,  which  con- 
vey it  into  the  gall-bladder.  As  this  fluid  is  required  only  during  intestinal 
digestion,  it  is  only  then  that  the  walls  of  the  gall-bladder  contract  and  dis- 
charge it  into  the  intestine. 

The  flow  of  bile  from  the  fiver  cells  into  the  gall-bladder  is  accomplished 


158  HUMAN  PHYSIOLOGY. 

by  the  inspiratory  movements  of  the  diaphragm,  and  by  the  contraction  of 
the  muscle-fibers  of  the  biliary  ducts,  as  well  as  the  vis  a  tergo  of  new-formed 
bile.  Any  obstacle  to  the  outflow  of  bile  into  the  intestine  leads  to  an  accumu- 
lation within  the  bile-ducts.  The  pressure  within  the  ducts  increasing 
beyond  that  of  the  blood  within  the  capillaries,  a  reabsorption  of  biliary 
matters  by  the  lymphatics  takes  place,  giving  rise  to  the  phenomena  of 
jaundice. 

The  bile  is  both  a  secretion  and  an  excretion;  it  contains  new  constituents, 
which  are  formed  only  in  the  substance  of  the  liver,  and  are  destined  to  play 
an  important  part  ultimately  in  nutrition;  it  contains  also  waste  ingredients, 
which  are  discharged  into  the  intestinal  canal  and  eliminated  from  the  body. 

The  Production  of  Glycogen. — In  addition  to  the  preceding  function, 
Bernard,  in  1848,  demonstrated  the  fact  that  the  liver,  during  life,  normally 
produces  a  substance  analogous  in  its  chemic  composition  to  starch,  which 
he  termed  glycogen;  also  that,  when  the  liver  is  removed  from  the  body,  and 
its  blood-vessels  are  thoroughly  washed  out,  after  a  few  hours  sugar  makes  its 
appearance  in  abundance.  The  sugar  can  also  be  shown  to  exist  in  the  blood 
of  the  hepatic  vein  as  well  as  in  a  decoction  of  the  liver  substance  by  means  of 
either  Trommer's  or  Fehling's  test,  even  when  the  blood  of  the  portal  vein 
does  not  contain  a  trace  of  sugar. 

Origin  and  Destination  of  Glycogen. — Glycogen  appears  to  be  formed 
in  the  liver  cells,  from  materials  derived  from  the  food,  whether  the  diet  be 
animal  or  vegetable,  though  a  larger  percentage  is  formed  when  the  animal 
is  fed  on  starchy  and  saccharine  than  when  fed  on  animal  food.  The  dex- 
trose, which  is  one  of  the  products  of  digestion,  is  absorbed  by  the  blood-vessels 
and  carried  directly  into  the  liver;  as  it  does  not  appear  in  the  urine,  as  it 
would  if  injected  at  once  into  the  general  circulation,  it  is  probable  that  it 
is  detained  in  the  liver,  dehydrated,  and  stored  up  as  glycogen.  The  change 
is  shown  by  the  following  formula: 

C6H12Os  —  H20  =C6H10O5. 

Dextrose.     Water.    Glycogen. 

The  glycogen  thus  formed  is  stored  up  in  the  hapatic  cells  for  the  future 
requirements  of  the  system.  When  sugar  is  needed  for  nutritive  purposes, 
the  glycogen  is  transformed  into  dextrose  by  the  agency  of  a  ferment. 

Glycogen,  when  obtained  from  the  liver,  is  an  amorphous,  starch-like 
substance,  of  a  white  color,  tasteless  and  colorless,  and  soluble  in  water; 
by  boiling  with  dilute  acids,  or  subjected  to  the  action  of  an  animal  ferment, 
it  is  easily  converted  into  dextrose.  When  an  excess  of  sugar  is  generated 
by  the  liver  out  of  the  glycogen,  dextrose  can  be  found  not  only  in  the  blood  of 
the  hepatic  vein,  but  also  in  other  portions  of  the  body;  under  these  circum- 


SKIN.  159 

stances  it  is  eliminated  by  the  kidneys,  appearing  in  the  urine,  constituting 
the  condition  of  glycosuria. 

Formation  of  Urea.— The  liver  is  now  regarded  by  many  physiologists 
as  the  principal  organ  concerned  in  urea  formation.  The  liver  normally 
contains  a  certain  amount  of  urea;  and  if  blood  be  passed  through  the  ex- 
cised liver  of  an  animal  which  has  been  in  full  digestion  when  killed,  a  large 
amount  of  urea  is  obtained.  The  clinical  evidence  proves  that  in  destructive 
diseases  of  the  liver  substance  there  is  at  once  a  falling-off  in  urea  elimination. 
Various  drugs  which  stimulate  liver  action  increase  the  amount  of  urea  in 
the  urine. 

The  antecedent  of  the  urea,  the  substances  out  of  which  the  liver  cells 
form  urea,  are  for  the  most  part  the  ammonium  salts,  the  carbonate  and 
carbamate,  which  are  brought  to  the  liver  by  the  blood  of  the  portal  vein. 
These  salts  are  formed  largely  in  the  intestinal  wall  out  of  the  amino  acids 
that  result  from  the  digestion  of  proteins.  It  is  also  very  probable  that  they 
arise  from  the  disintegration  of  proteins  in  other  portions  of  the  body. 

Influence  of  the  Nerve  System. — The  nerve  system  directly  controls 
the  functional  activity  of  the  liver,  and  more  especially  its  glycogenic  function. 
It  was  discovered  by  Bernard  that  puncture  of  the  medulla  oblongata  is 
followed  by  so  enormous  a  production  of  sugar  that  it  is  at  once  excreted  by 
the  kidneys,  giving  rise  to  diabetic  or  saccharine  urine.  This  part  of  the 
medulla  is,  however,  the  vaso-motor  center  for  the  blood-vessels  of  the  liver. 
Destruction  of  this  center,  or  injury  to  the  vaso-motor  nerves  emanating  from 
it  in  any  part  of  their  course,  is  followed  at  once  by  dilatation  of  the  hepatic 
blood-vessels,  slowing  of  the  blood-current,  a  profound  disturbance  of  the 
normal  relation  existing  between  the  blood  and  liver-cells,  and  a  production 
of  sugar.  Many  of  the  hepatic  vaso-motor  nerves  may  be  traced  down  to  the 
cord  as  far  as  the  lumbar  region,  while  others  leave  the  cord  high  up  in  the 
neck  and  enter  the  cervical  ganglia  of  the  sympathetic,  and  so  reach  the 
liver.  Injury  to  the  sympathetic  ganglia  is  often  followed  by  diabetes. 
Peripheral  stimulation  of  various  nerves — e.  g.,  sciatic,  pneumogastric,  de- 
pressor nerve, — as  well  as  the  direct  action  of  many  drugs,  impair  or  depress 
the  hepatic  vaso-motor  center  and  so  give  rise  to  diabetes. 

SKIN. 

The  skin,  the  external  investment  of  the  body,  is  a  most  complex  and  im- 
portant structure,  serving — 

1.  As  a  protective  covering. 

2.  As  an  organ  for  tactile  sensibility. 

3.  As  an  organ  for  the  elimination  of  excrementitious  matters. 


160  HUMAN  PHYSIOLOGY. 

The  amount  of  skin  investing  the  body  of  a  man  of  average  size  is  about 
twenty  feet,  and  varies  in  thickness,  in  different  situations,  from  ^  to  tI~q  of 
an  inch. 

The  skin  consists  of  two  principal  layers — viz.,  a  deeper  portion  the 
corium,  and  a  superficial  portion,  the  epidermis. 

The  corium,  or  cutis  vera,  may  be  subdivided  into  a  reticulated  and  a 
papillary  layer.  The  former  is  composed  of  white  fibrous  tissue,  non-striated 
muscle-fibers,  and  elastic  tissue,  interwoven  in  every  direction,  forming  an 
areolar  network,  in  the  meshes  of  which  are  deposited  masses  of  fat,  and  a 
structureless,  amorphous  matter;  the  latter  is  formed  mainly  of  club-shaped 
elevations  or  projections  of  the  amorphous  matter,  constituting  the  papilla; 
they  are  most  abundant  and  well  developed  upon  the  palms  of  the  hands  and 
upon  the  soles  of  the  feet;  they  average  T^o  of  an  inch  in  length,  and  may 
be  simple  or  compound;  they  are  well  supplied  with  nerves,  blood-vessels, 
and  lymphatics. 

The  epidermis,  or  scarf  skin,  is  an  extravascular  structure,  a  product 
of  the  true  skin,  and  is  composed  of  several  layers  of  cells.  It  may  be  divided 
into  two  layers:  the  rete  mucosum,  or  the  Malpighian  layer,  and  the  horny  or 
corneous. 

The  former  is  closely  adherent  to  the  papillary  layer  of  the  true  skin,  and 
is  composed  of  large  nucleated  cells,  the  lowest  layer  of  which,  the  "prickle 
cells,"  contains  pigment-granules,  which  give  to  the  skin  its  varying  tints 
in  different  individuals  and  in  different  races  of  men;  the  more  superficial 
cells  are  large,  colorless,  and  semi-transparent.  The  latter,  the  corneous 
layer,  is  composed  of  flattened  cells,  which,  from  their  exposure  to  the  atmos- 
phere, are  hard  and  horny  in  texture;  it  varies  in  thickness  from  |  of  an 
inch  on  the  palms  of  the  hands  and  soles  of  the  feet  to  9^0  of  an  inch  in  the 
external  auditory  canal. 

APPENDAGES  OF  THE  SKIN. 

Hairs  are  found  in  almost  all  portions  of  the  body,  and  can  be  divided 
into — 

1.  Long,  soft  hairs,  on  the  head. 

2.  Short,  stiff  hairs,  along  the  edges  of  the  eyelids  and  nostrils. 

3.  Soft,  downy  hairs  on  the  general  cutaneous  surface. 

They  consist  of  a  root  and  a  shaft.  The  latter  is  oval  in  shape  and  about 
jjjq  of  an  inch  in  diameter;  it  consists  of  fibrous  tissue,  covered  externally 
by  a  layer  of  imbricated  cells,  and  internally  by  cells  containing  granular 
and  pigment  material. 


SKIN.  l6l 

The  root  of  the  hair  is  embedded  in  the  hair-follicle,  formed  by  a  tubular 
depression  of  the  skin,  extending  nearly  through  to  the  subcutaneous  tissue; 
its  walls  are  formed  by  the  layers  of  the  corium,  covered  by  epidermic  cells. 
At  the  bottom  of  the  follicle  is  a  papillary  projection  of  amorphous  matter, 
corresponding  to  a  papilla  of  the  true  skin,  containing  blood-vessels  and 
nerves,  upon  which  the  hair-root  rests.  The  investments  of  the  hair-roots 
are  formed  of  epithelial  cells,  constituting  the  internal  and  external 
root-sheaths. 

The  hair  protects  the  head  from  the  heat  of  the  sun  and  from  the  cold, 
retains  the  heat  of  the  body,  prevents  the  entrance  of  foreign  matter  into 
the  lungs,  nose,  ears,  etc.  The  color  is  due  to  pigment  matter.  In  old  age 
the  hair  becomes  more  or  less  whitened. 

The  sebaceous  glands,  embedded  in  the  true  skin,  are  simple  and  com- 
pound racemose  glands,  opening,  by  a  common  excretory  duct,  upon  the 
surface  of  the  epidermis  or  into  the  hair-follicle.  They  are  found  in  all  por- 
tions of  the  body,  most  abundantly  in  the  face,  and  are  formed  by  a  delicate, 
structureless  membrane,  lined  by  flattened  polyhedral  cells.  The  sebaceous 
glands  secrete  a  peculiar  oily  matter  (the  sebum),  by  which  the  skin  is  lubri- 
cated and  the  hairs  are  softened;  it  is  quite  abundant  in  the  region  of  the  nose 
and  forehead,  which  often  presents  a  greasy,  glistening  appearance;  it 
consists  of  water,  mineral  salts,  fatty  globules,  and  epithelial  cells. 

The  vernix  caseosa,  which  frequently  covers  the  surface  of  the  fetus  at 
birth,  consists  of  the  residue  of  the  sebaceous  matter,  containing  epithelial 
cells  and  fatty  matters;  it  seems  to  keep  the  skin  soft  and  supple,  and  guards 
it  from  the  effects  of  the  long-continued  action  of  the  amniotic  water. 

The  sudoriparous  glands  excrete  the  sweat.  They  consist  of  a  mass 
or  coil  of  a  tubular  gland  duct,  situated  in  the  derma  and  in  the  subcutaneous 
tissue,  average  ~\  of  an  inch  in  diameter,  and  are  surrounded  by  a  rich 
plexus  of  capillary  blood-vessels.  From  this  coil  the  duct  passes  in  a  straight 
direction  up  through  the  skin  to  the  epidermis,  where  it  makes  a  few  spiral 
turns  and  opens  obliquely  upon  the  surface.  The  sweat-glands  consist  of  a 
delicate  homogeneous  membrane  lined  by  epithelial  cells,  whose  function  is  to 
extract  from  the  blood  the  elements  existing  in  the  perspiration. 

The  glands  are  very  abundant  all  over  the  cutaneous  surface — as  mam- 
as 3528  to  the  square  inch,  according  to  Erasmus  Wilson. 

The  perspiration  is  an  excrementitious  fluid,  clear,  colorless,  almost 
odorless,  slightly  acid  in  reaction,  with  a  specific  gravity  of  1003  to  1004. 

The  total  quantity  of  perspiration  excreted  daily  has  been  estimated 
at  about  two  pounds,  though  the  amount  varies  with  the  nature  of  the  food 
and  drink,  exercise,  external  temperature,  season,  etc. 
11 


1 62  HUMAN  PHYSIOLOGY. 

The  elimination  of  the  sweat  is  not  intermittent,  but  continuous:  it  takes 
place  so  gradually  that  as  fast  as  it  is  formed  it  passes  off  by  evaporation  as 
insensible  perspiration.  Under  exposure  to  great  heat  and  exercise  the 
evaporation  is  not  sufficiently  rapid,  and  it  appears  as  sensible  perspiration. 

COMPOSITION   OF   SWEAT. 

Water 995-573 

Urea o .  043 

Fatty  matters 0.014 

Alkaline  lactates o  .317 

Alkaline  sudorates 1 .  562 

Inorganic  salts 2 .491 


1,000.000 


Urea  is  a  constant  ingredient. 

Carbonic  acid  is  also  exhaled  from  the  skin,  the  amount  being  about  2  <io 
of  that  from  the  lungs. 

Perspiration  regulates  the  temperature  and  removes  waste  matters  from 
the  blood;  it  is  so  important  that  if  elimination  be  prevented,  death  occurs  in 
a  short  time. 

Influence  of  the  Nerve  System. — The  secretion  of  sweat  is  regulated 
by  the  nerve  system.  Here,  as  in  the  secreting  glands,  the  fluid  is  formed 
from  material  in  the  lymph-spaces  surrounding  the  gland.  Two  sets  of 
nerves  are  concerned — viz.,  vasomotor,  regulating  the  blood-supply;  and 
secretor,  stimulating  the  activities  of  the  gland  cells.  Generally  the  two 
conditions,  increased  blood  flow  and  increased  glandular  action,  coexist. 
At  times  profuse  clammy  perspiration  occurs,  with  diminished  blood  flow. 

The  dominating  sweat-center  is  located  in  the  medulla,  though  subordinate 
centers  are  present  in  the  cord.  The  secretory  fibers  reach  the  perspiratory 
glands  of  the  head  and  face  through  the  cervical  sympathetic;  of  the  arms, 
through  the  thoracic  sympathetic,  ulnar,  and  radial  nerves;  of  the  leg,  through 
the  abdominal  sympathetic  and  sciatic  nerves. 

The  sweat-center  is  excited  to  action  by  mental  emotions,  increased 
temperature  of  blood  circulating  in  the  medulla  and  cord,  increased  venosity 
of  blood,  many  drugs,  rise  of  external  temperature,  exercise,  etc. 


THE  CENTRAL  ORGANS  OF  THE  NERVE  SYSTEM 
AND  THEIR  NERVES. 

The  central  organs  of  the  nerve  system  are  the  encephalon  and  the  spinal 
cord,  lodged  within  the  cavity  of  the  cranium  and  the  cavity  of  the  spinal 
column  respectively.  The  general  shape  of  these  two  portions  of  the  nerve 
system  conespond  with  that  of  the  cavities  in  which  they  are  contained. 
The  encephalon  is  broad  and  ovoid,  the  spinal  cord  is  narrow  and  elongated. 

The  encephalon  is  subdivided  by  deep  fissures  into  four  distinct,  though 
closely  related  portions:  viz.,  (i)  the  cerebrum,  the  large  ovoid  mass  occupy- 
ing the  entire  upper  part  of  the  cranial  cavity;  (2)  the  cerebellum,  the  wedge- 
shaped  portion  placed  beneath  the  posterior  part  of  the  cerebrum  and  lodged 
within  the  cerebellar  fossae;  (3)  the  isthmus  of  the  encephalon,  the  more  or 
less  pyramidal-shaped  portion  connecting  the  cerebrum  and  cerebellum  with 
each  other  and  both  with  (4)  the  medulla  oblongata. 

The  spinal  cord  is  narrow  and  cylindric  in  shape.  It  occupies  the  spinal 
canal  as  far  down  as  the  second  or  third  lumbar  vertebra. 

The  nerves  in  relation  with  the  central  organs  of  the  nerve  system  are  the 
encephalic  or  cranial  and  the  spinal  nerves. 

The  encephalic  nerves  are  twelve  in  number  on  each  side  of  the  median 
line.  Because  of  the  fact  that  they  pass  through  foramina  in  the  walls  of  the 
cranium  they  are  usually  termed  cranial  nerves. 

The  spinal  nerves  are  thirty-one  in  number  on  each  side  of  the  cord. 

The  cranial  and  spinal  nerves  are  ultimately  distributed  to  all  the  struc- 
tures of  the  body — e.  g.,  the  general  periphery,  and  for  this  reason  they  are 
collectively  known  as  the  peripheral  organs  of  the  nerve  system. 

The  central  organs  of  the  nerve  system  are  supported  and  protected  by 
three  membranes  named,  in  their  order  from  without  inward,  as  the  dura 
mater,  the  arachnoid  and  the  pia  mater. 

The  dura  mater,  the  outermost  of  the  three,  is  a  tough  membrane,  com- 
posed of  white  fibrous  tissue  arranged  in  bundles,  which  interlace  in  every 
direction.  In  the  cranial  cavity  it  lines  the  inner  surface  of  the  bones,  and 
is  attached  to  the  edge  of  the  foramen  magnum;  it  sends  processes  inward, 
forming  the  falx  cerebri,  falx  cerebelli,  and  tentorium  cerebelli,  supporting 
and  protecting  parts  of  the  brain.  In  the  spinal  canal  it  loosely  invests 
the  cord,  and  is  separated  from  the  walls  of  the  canal  by  areolar  tissue. 

163 


164  HUMAN  PHYSIOLOGY. 

The  arachnoid,  the  middle  membrane,  is  a  delicate  serous  structure 
which  envelops  the  brain  and  cord,  forming  the  visceral  layer,  and  is  then 
reflected  to  the  inner  surface  of  the  dura  mater,  forming  the  parietal  layer. 
Between  the  two  layers  there  is  a  small  quantity  of  fluid  which  prevents 
friction  by  lubricating  the  two  surfaces. 

The  pia  mater,  the  most  internal  of  the  three,  composed  of  areolar  tissue 
and  blood-vessels,  covers  the  entire  surface  of  the  brain  and  cord,  to  which 
it  is  closely  adherent,  dipping  down  between  the  convolutions  and  fissures. 
It  is  exceedingly  vascular,  sending  small  blood-vessels  some  distance  into  the 
brain  and  cord. 

The  cerebro-spinal  fluid  occupies  the  subarachnoid  space  and  the  general 
ventricular  cavities  of  the  brain,  which  communicate  by  an  opening  (the 
foramen  of  Magendie)  in  the  pia  mater,  at  the  lower  portion  of  the  fourth 
ventricle.  This  fluid  is  clear,  transparent,  alkaline,  possesses  a  salty  taste 
and  has  a  low  specific  gravity;  it  is  composed  largely  of  water,  traces  of  al- 
bumin, glucose,  and  mineral  salts.  The  quantity  is  estimated  from  two  to 
four  fluidounces. 

The  function  of  the  cerebro-spinal  fluid  is  to  protect  the  brain  and  cord  by 
preventing  concussion  from  without;  by  being  easily  displaced  into  the  spinal 
canal,  prevents  undue  pressure  and  insufficiency  of  blood  to  the  brain. 

SPINAL  CORD. 

The  spinal  cord  varies  from  sixteen  to  eighteen  inches  in  length;  is  | 
of  an  inch  in  thickness,  weighs  ij  ounces,  and  extends  from  the  atlas  to 
the  second  lumbar  vertebra,  terminating  in  the  filum  terminale.  It  is  cylin- 
dric  in  shape,  and  presents  an  enlargement  in  the  lower  cervical  and  lower 
dorsal  regions,  corresponding  to  the  origin  of  the  nerves  which  are  distrib- 
uted to  the  upper  and  lower  extremities.  The  cord  is  divided  into  two 
lateral  halves  by  the  anterior  and  posterior  fissures.  It  is  composed  of  both 
white  or  -fibrous  and  gray  or  vesicular  matter,  the  former  occupying  the  ex- 
terior of  the  cord,  the  latter  the  interior,  where  it  is  arranged  in  the  form  of 
two  crescents,  one  in  each  lateral  half,  united  by  the  central  mass,  the  gray 
commissure;  the  white  matter  being  united  in  front  by  the  white  commissure. 

Structure  of  the  Gray  Matter. — The  gray  matter  is  arranged  in  the 
form  of  two  crescents,  united  by  a  commissural  band,  formng  a  figure  re- 
sembling the  letter  H.  Each  crescent  presents  a  ventral  and  a  dorsal 
horn.  The  center  of  the  commissure  presents  a  canal  which  extends  from 
the  fourth  ventricle  downward  to  the  filum  terminale.  The  ventral  horn 
is  short  and  broad  and  does  not  extend  to  the  surface.  The  dorsal  horn 
is  narrow  and  elongated  and  extends  quite  to  the  surface.     It  is  covered  and 


SPINAL    CORD. 


I65 


capped  by  the  substantia  gelatinosa.  The  gray  matter  consists  primarily  of  a 
framework  of  fine  connective  tissue,  supporting  blood-vessels,  lymphatics, 
medullated  and  non-medullated  nerve-fibers,  and  groups  of  nerve-cells. 


Fig.  21. — Superior,  Middle,  and  Inferior  Portions  of  Spinal  Cord. 
1.  Floor  of  fourth  verticle.  2.  Superior  cerebellar  peduncle.  3.  Middle  cerebellar 
peduncle.  4.  Inferior  cerebellar  peduncle.  5.  Enlargement  at  upper  extremity  of 
postero-median  column.  6.  Glosso-pharyngeal  nerve.  7.  Vagus.  8.  Spinal  acces- 
sory. 9,  9,  9,  9.  Ligamentum  denticulatum.  10,  10,  10,  10.  Posterior  roots  of  spinal 
nerves,  n,  n,  n,  11.  Postero-lateral  fissure.  12,  12,  12,  12.  Ganglia  of  posterior 
roots.  13,  13.  Anterior  roots.  14.  Division  of  united  roots  into  anterior  and  pos- 
terior nerves.  15.  Terminal  extremity  of  cord.  16,  16.  Filum  terminale.  17,  17. 
Cauda  equina.  I,  VIII.  Cervical  nerves.  I,  XII.  Dorsal  nerves.  I,  V.  Lumbar 
nerves.     I,  V.  Sacral  nerves. — (Sappey.) 

The  nerve-cells  are  arranged  in  groups,  which  extend  for  some  distance 
throughout  the  cord,  forming  columns  more  or  less  continuous.     The  first 
group  is  situated  in  the  ventral  horn,  the  cells  of  which  are  large,  multipolar,* 
and  connected  with  the  ventral  roots  of  the  spinal  nerves,  and  are  supposed 
to  be  motor  in  function.     The  second  group  is  situated  in  the  dorsal  horn, 


i66 


HUMAN  PHYSIOLOGY. 


the  cells  of  which  are  spindle-shaped,  and  from  their  relation  to  the  posterior 
roots  are  supposed  to  be  sensory  in  function.  The  third  group  is  situated  in 
the  lateral  aspect  of  the  gray  matter,  and  is  quite  separate  and  distinct, 
except  in  the  lumbar  and  cervical  enlargements,  where  it  blends  with  those 
of  the  ventral  horn.  A  fourth  group  is  situated  at  the  inner  base  of  the 
dorsal  horn;  it  begins  about  the  seventh  or  eighth  cervical  nerve  and 
extends  downward  to  the  second  or  third  lumbar,  being  most  prominent  in 
the  dorsal  region.     This  column  is  known  as  Clark's  vesicular  column. 

Structure  of  the  White  Matter. — The  white  matter  surrounding  each 
lateral  half  of  the  cord  is  made  up  of  nerve  fibers,  some  of  which  are  con- 
tinuations for  the  nerves  which  enter 
a.       .  the    cord,   while  others    are    derived 

from    different   sources.      It  is   sub- 
\  ^-^-%^W*C-^  divided  into — 

A  ventral  column,  comprising  that 

portion   between  the  ventral  roots 

and    the  ventral  fissure,    which  is 

again  subdivided  into  two  parts: 

(a)  An  inner  portion,  bordering 

the  ventral  median  fissure,  the 

direct  pyramidal  tract,  or  column 

of  Tiirck;    it  contains    motor 

fibers  which  do  not  decussate, 

and  which  extend  as  far  down 

as   the  middle   of    the    dorsal 


Fig.  22. — Scheme  of  the  Conducting 
Path  in.  the  Spinal  Cord  at  the  Third 
Dorsal  Nerve. — (Landois.) 
_  The  black  part  is  the  gray  matter.  v. 
Ventral,  hw,  dorsal  root.  a.  Direct,  and 
g,  g,  crossed,  pyramidal  tracts,  b.  Ven- 
tral column,  ground  bundle.  c.  Goll's 
column.  d.  Postero-external  column. 
e,  e,  and  f,  f.  Mixed  lateral  paths.  h,  h. 
Direct  cerebellar  tracts. 


region. 
(b)   An  outer  portion,  surrounding 
the  ventral  cornua,   known  as 
the  ventral  root  zone,  composed 
of     short,    longitudinal    fibers 
which  serve  to  connect  different  segments  of  the  spinal  cord. 
A  lateral  column,  the  portion  between  the  ventral  and  dorsal  roots,  which 
is  divisible  into — 

(a)  The  crossed  pyramidal  tract,  occupying  the  dorsal  portion  of  the 
lateral  column,  and  containing  all  those  fibers  of  the  motor  tract 
which  have  decussated  at  the  medulla  oblongata;  it  is  composed  of 
longitudinally  running  fibers,  which  are  connected  with  the  multi- 
polar nerve-cells  of  the  ventral  cornua. 

(b)  The  direct  cerebellar  tract,  situated  upon  the  surface  of  the  lateral 
column,    consisting  of   longitudinal   fibers   which    terminate   in    the 


SPINAL   CORD.  167 

cerebellum;  it  first  appears  in  the  lumbar  region,  and  increases  in 
thickness  as  it  passes  upward, 
(c)  The  ventral  trad,  lying  just  posterior  to  the  ventral  cornua. 

3.  A  dorsal  column,  the  portion  included  between  the  dorsal  roots  and  the 
dorsal  fissure,  also  divisible  into  two  portions: 

(a)  An  inner  portion,  the  poster -o-internal  column,  or  the  column  of  Goll, 
bordering  the  dorsal  median  fissure,  and 

(b)  An  external   portion,  the   poster o-extemal  column,  the    column  of 
Burdach,  lying  just  behind  the  dorsal  roots. 

The  two  portions  of  the  dorsal  column  are  composed  of  long  and  short 
commissural  fibers,  which  connect  different  segments  of  the  spinal  cord. 

The  Relation  of  the  Spinal  Nerves  to  the  Spinal  Cord. — The  spinal 
nerves  present  near  the  spinal  cord  two  divisions  which  from  their  connection 
with  the  anterior  or  ventral  and  the  posterior  or  dorsal  surfaces  are  known 
as  the  anterior  or  ventral  and  posterior  or  dorsal  roots.  The  ventral  roots 
are  composed  of  nerve-fibers  which  have  their  origin  in  the  nerve-cells  in 
the  anterior  horns  of  the  gray  matter.  The  dorsal  roots  are  composed  of 
nerve-fibers  which  have  their  origin  in  the  nerve-cells  in  the  spinal  ganglia. 
After  entering  the  cord  some  of  the  posterior  or  dorsal  root-fibers  arborize 
around  nerve-cells  in  the  gray  matter  at  the  same  level;  others  pass  obliquely 
upward  through  the  posterior  white  columns  as  far  as  the  nucleus  gracilis 
and  the  nucleus  cuneatus  around  the  nerve-cells  of  which  they  terminate. 

FUNCTIONS  OF  THE  SPINAL  CORD. 

The  spinal  cord,  by  virtue  of  its  contained  nerve-cells  and  nerve-fibers, 
may  be  regarded  as  composed  of — 

1.  Independent  nerve  centers  each  of  which  has  a  special  function;  and — 

2.  Of  conducting  paths  by  which  these  centers  are  brought  into  relation  with 
one  another  and  with  the  cerebrum  and  its  subordinate  or  underlying  parts. 

1.  As  an  Independent  Nerve  Center. 

The  spinal  cord,  by  virtue  of  its  contained  nerve-cells,  is  capable  of  trans- 
forming afferent  nerve  impulses  arriving  through  the  afferent  nerves  into 
efferent  impulses,  which  are  reflected  outward  through  efferent  nerves  to 
muscles,  producing  motion;  to  glands,  exciting  secretion;  to  blood-vessels, 
changing  their  caliber.  All  such  actions  taking  place  independent  of  either 
sensation  or  volition  are  termed  reflex  actions.  The  mechanism  involved  in 
every  reflex  action  consists  of  a  receptive  surface,  an  afferent  nerve,  an 
emissive  center,  an  efferent  nerve,  and  a  responsive  organ,  muscle,  gland,  or 
blood-vessels. 


1 68  HUMAN  PHYSIOLOGY. 

The  reflex  excitability  of  the  cord  may  be — 

i.  Increased  by  disease  of  the  lateral  columns,  by  the  administration  of 
strychnin,  and,  in  frogs,  by  a  separation  of  cord  from  the  brain,  the  latter 
apparently  exerting  an  inhibitor  influence  over  the  former  and  depressing 
its  reflex  activity. 

2.  Decreased  by  destructive  lesion  of  the  cord — e.  g.,  locomotor  ataxia, 
atrophy  of  the  anterior  cornua — the  administration  of  various  drugs,  and, 
in  the  frog,  by  irritation  of  certain  regions  of  the  brain.  When  the  cere- 
brum alone  is  removed  and  the  optic  lobes  are  stimulated,  the  time  elapsing 
between  the  application  of  an  irritant  to  a  sensor  surface  and  the  resulting 
movement  will  be  considerably  prolonged,  the  optic  lobes  (Setschenow's 
center)  apparently  generating  impulses  which,  descending  the  cord,  retard 
its  reflex  movement. 

Classification  of  Reflex  Movement. — They  may  be  divided  into  four 
groups,  according  to  the  route  through  which  the  afferent  and  efferent 
impulses  pass: 

i.  Those  normal  reflex  acts  (e.  g.,  deglutition,  coughing,  sneezing,  walking, 
etc.)  and  pathologic  reflex  acts  (e.  g.,  tetanus,  vomiting,  epilepsy)  which 
take  place  both  afferently  and  efferently  through  spinal  nerves. 

2.  Reflex  acts  which  take  place  in  an  afferent  direction  through  a  cerebro- 
spinal sensor  nerve,  and  in  an  efferent  direction  through  a  sympathetic 
motor  nerve — e.  g.,  the  normal  reflex  acts,  which  give  rise  to  most  of  the 
secretions,  pallor  of  the  skin  and  blushing,  certain  movements  of  the  iris, 
certain  modifications  in  the  beat  of  the  heart;  the  pathologic  reflexes, 
which,  on  account  of  the  difficulty  in  explaining  their  production,  are 
termed  metastatic — e.  g.,  ophthalmia,  coryza,  orchitis,  which  depend  on  a 
reflex  hyperemia;  amaurosis,  paralysis,  paraplegia,  etc.,  due  to  a  reflex 
anemia. 

3.  Reflex  movements  in  which  the  afferent  impulse  passes  through  a  sym- 
pathetic nerve,  and  the  efferent  through  a  cerebro-spinal  nerve;  most  of 
these  phenomena  are  pathological — e.  g.,  convulsions  from  intestinal  irrita- 
tion produced  by  the  presence  of  worms,  eclampsia,  hysteria,  etc. 

Laws  of  Reflex  Action  (Pfliiger). 

1.  Law  of  Unilaterality. — If  a  feeble  irritation  be  applied  to  one  or  more 
sensory  nerves,  movement  takes  place  usually  on  one  side  only,  and  that 
the  same  side  as  the  irritation. 

2.  Law  of  Symmetry. — If  the  irritation  becomes  sufficiently  intense,  motor 
reaction  is  manifested,  in  addition,  in  corresponding  muscles  of  the  oppo- 
site side  of  the  body. 


SPINAL   CORD.  169 

3.  Law  of  Intensity. — Reflex  movements  are  usually  more  intense  on  the  side 
of  the  irritation;  at  times  the  movements  of  the  opposite  side  equal  them 
in  intensity,  but  they  are  usually  less  pronounced. 

4.  Law  of  Radiation. — If  the  excitation  still  continues  to  increase,  it  is  prop- 
agated upward,  and  motor  reaction  takes  place  through  centrifugal 
nerves  coming  from  segments  of  the  cord  higher  up. 

5.  Law  of  Generalization. — When  the  irritation  becomes  very  intense,  it  is 
propagated  in  the  medulla  oblongata;  motor  reaction  then  becomes  general, 
and  it  is  propagated  up  and  down  the  cord,  so  that  all  the  muscles  of  the 
body  are  thrown  into  action,  the  medulla  oblongata  acting  as  a  focus 
whence  radiate  all  reflex  movements. 

Special  Reflex  Movements. 

Among  the  reflexes  connected  with  the  more  superficial  portions  of  the  body 
there  are  some  which  are  so  frequently  either  exaggerated  or  diminished  in 
pathologic  lesions  of  the  spinal  cord  that  their  study  affords  valuable  indica- 
tions as  to  the  seat  and  character  of  the  lesions.     They  may  be  divided  into — 

1.  Skin  or  superficial,  and 

2.  Tendon  or  deep  reflexes. 

The  skin  reflexes,  characterized  by  contraction  of  underlying  muscles, 
are  induced  by  irritation  of  the  skin — e.  g.,  pricking,  pinching,  scratching, 
etc.     The  following  are  the  principal  skin  reflexes: 

1.  Plantar  reflex,  consisting  of  contraction  of  the  muscles  of  the  foot,  induced 
by  stimulation  of  the  sole  of  the  foot;  it  involves  the  integrity  of  the  reflex 
arc  through  the  lower  end  of  the  cord. 

2.  Gluteal  reflex,  consisting  of  contraction  of  the  glutei  muscles  when  the 
skin  over  the  buttock  is  stimulated;  it  takes  place  through  the  segments 
giving  origin  to  the  fourth  and  fifth  lumbar  nerves. 

3.  Cremasteric  reflex,  consisting  of  a  contraction  of  the  cremaster  muscle 
and  a  retraction  of  the  testicle  toward  the  abdominal  ring  when  the  skin 
on  the  inner  side  of  the  thigh  is  stimulated;  it  depends  upon  the  integrity 
of  the  segments  giving  origin  to  the  first  and  second  lumbar  nerves. 

4.  Abdominal  reflex,  consisting  of  a  contraction  of  the  abdominal  muscles 
when  the  skin  upon  the  side  of  the  abdomen  is  gently  scratched;  its  pro- 
duction requires  the  integrity  of  the  spinal  segments  from  the  eighth  to  the 
twelfth  thoiacic  nerves. 

5.  Epigastric  reflex,  consisting  of  a  slight  muscular  contraction  in  the  neigh- 
borhood of  the  epigastrium  when  the  skin  between  the  fourth  and  sixth 
ribs  is  stimulated;  it  requires  the  integrity  of  the  cord  between  the  fourth 
and  seventh  thoracic  nerves. 

6.  Scapular  reflex    consisting    of    a    contraction    of    the    scapular  muscles 


170  HUMAN  PHYSIOLOGY. 

when  the  skin  between  the  scapulae  is  stimulated;  it  depends  upon  the 

integrity  of  the  cord  between  the  fifth  cervical  and  third  thoracic  nerves. 

The  superficial  reflexes,  though  variable,  are  generally  present  in  health. 
They  are  increased  or  exaggerated  when  the  gray  matter  of  the  cord  is 
abnormally  excited,  as  in  tetanus,  strychnia-poisoning,  and  disease  of  the 
lateral  columns. 

The  tendon  reflexes,  characterized  by  the  contraction  of  a  muscle,  are  also  of 
much  value  in  the  diagnosis  of  lesions  of  the  spinal  cord  and  are  elicited  by  a 
sharp  blow  on  a  tendon.     The  following  are  the  principal  tendon  reflexes: 

1.  Patellar  reflex,  or  knee-jerk,  consisting  of  a  contraction  of  the  extensor 
muscles  of  the  thigh  when  the  ligamentum  patellae  is  struck  between  the 
patella  and  tibia.  This  reflex  is  best  observed  when  the  legs  are  freely 
hanging  over  the  edge  of  a  table.  The  patellar  reflex  is  generally  present 
in  health,  being  absent  in  only  two  percent;  it  is  greatly  exaggerated  in 
lateral  sclerosis  and  in  descending  degeneration  of  the  cord;  it  is  absent 
in  locomotor  ataxia  and  in  atrophic  lesions  of  the  anterior  gray  cornua. 

2.  Ankle- jerk  or  Ankle  Reflex. — If  the  entensor  muscles  of  the  leg  be  placed 
upon  the  stretch  and  the  tendo  Achillis  be  sharply  struck,  a  quick  extension 
of  the  foot  will  take  place. 

3.  Ankle-clonus. — This  consists  of  a  series  of  rhythmic  reflex  contractions 
of  the  gastrocnemius  muscle,  varying  in  frequency  from  six  to  ten  a 
second.  To  elicit  this  reflex,  pressure  is  made  upon  the  sole  of  the  foot 
so  as  suddenly  and  energetically  to  flex  the  foot  at  the  ankle,  thus  putting 
the  tendo  Achillis  and  the  gastrocnemius  muscle  on  the  stretch.  The 
rhythmic  movements  thus  produced  continue  so  long  at  the  tension,  with- 
in limits,  is  maintained.  Ankle-clonus  is  never  present  in  health,  but  is 
very  marked  in  lateral  sclerosis  of  the  cord. 

The  toe  reflex,  peroneal  reflex,  and  wrist  reflex  are  also  present  in  sclerosis 
of  the  lateral  columns  and  in  the  late  rigidity  of  hemiplegia. 

Special  Nerve  Centers  in  Spinal  Cord. — Throughout  the  spinal  cord 
there  are  a  number  of  spinal  nerve  centers,  capable  of  being  excited  reflexly 
and  of  producing  complex  coordinated  movements.  Though  for  the  most 
part  independent  in  action,  they  are  subject  to  the  controlling  influences  of 
the  medulla  and  brain. 

1.  Ciliospinal  center,  situated  in  the  cord  between  the  lower  cervical  and 
the  third  dorsal  vertebra.  It  is  connected  with  the  dilatation  of  the  pupil 
through  fibers  which  emerge  in  this  region  and  enter  the  cervical  sympa- 
thetic. Stimulation  of  the  cord  in  this  locality  causes  dilatation  of  the 
pupil  on  the  same  side;  destruction  of  the  cord  is  followed  by  contraction 
of  the  pupil. 


SPINAL   CORD.  171 

2.  Genitospinal  center,  situated  in  the  lower  part  of  the  cord.  This  is  a 
complex  center,  and  comprises  a  series  of  subordinate  centers  for  the  control 
of  the  muscular  movements  involved  in  the  acts  of  defecation,  micturition, 
and  ejaculation  of  semen,  and  of  the  movements  of  the  uterus  during 
parturition,  etc. 

3.  Vaso-motor  centers,  giving  origin  to  both  vaso-constrictor  and  vaso- 
dilatator fibers,  which  are  distributed  throughout  the  cord  between  the 
first  thoracic  and  third  lumbar  nerves. 

Though  acting  reflexly,  they  are  under  the  dominating  influence  of  the 
center  in  the  medulla. 

4.  Sweat-centers  are  also  present  in  various  parts  of  the  cord. 

2 .  As  a  Conductor. 

The  white  matter  of  the  spinal  cord  consists  of  nerve-fibers,  the  specific 
function  of  which  is, 

1.  To  conduct  nerve  impulses  from  one  segment  of  the  cord  to  another. 

2 .  To  conduct  nerve  impulses  coming  from  the  encephalon  to  the  spinal  cord 
segments. 

3.  To  conduct  nerve  impulses  coming  to  the  cord  through  afferent  nerves, 
directly  or  indirectly  to  the  encephalon. 

Intersegmental  Conduction. — The  spinal  cord  consists  of  a  series  of  phys- 
iologic segments  each  of  which  has  a  special  function  and  is  associated  through 
its  related  spinal  nerve  with  a  definite  segment  of  the  body.  For  the  har- 
monious cooperation  and  coordination  of  all  the  spinal  segments  it  is  essential 
that  they  should  be  united  by  commissural  or  associative  fibers.  The  cord 
thus  becomes  capable  of  complex  and  purposive  reflex  actions. 

Encephalo-spinal,  or  Motor  Conduction. — The  nerve-fibers  which 
conduct  volitional  impulses  from  the  brain  downward  to  the  ventral  cornua 
arise  in  the  motor  centers  of  the  cerebrum;  they  then  pass  downward  through 
the  corona  radiata,  the  internal  capsule,  the  inferior  portions  of  the  crura 
cerebri,  the  pons  Varolii,  to  the  medulla  oblongata,  where  the  motor  tract  of 
each  side  divides  into  two  portions,  viz.: 

1.  The  larger,  containing  ninety-one  to  ninety-seven  per  cent,  of  the  fibers, 
which  decussates  at  the  lower  border  of  the  medulla  and  passes  down  in  the 
lateral  column  of  the  opposite  side,  and  constitutes  the  crossed  pyramidal  tract. 

2.  The  smaller,  containing  three  to  nine  per  cent,  of  the  fibers,  does  not  at 
once  decussate,  but  passes  down  the  ventral  column  of  the  same  side,  and 
constitutes  the  direct  pyramidal  tract,  or  the  column  of  Tiirck.  At  a  lower 
level  this  tract  also  decussates  or  crosses  over  to  the  opposite  side  of  the  cord. 
The  fibers  of  both  the  crossed  and  the  direct  pyramidal  tracts  come  into 


172 


HUMAN  PHYSIOLOGY. 


relation  by  their  terminal  branches  with  the  nerve-cells  in  the   ventral  cornu 
of  the  gray  matter  of  the  opposite  side  of  the  cord.     (Fig.  23.) 


^A  R 

Fig.  23.— Course  of  the  Fibers  for  Voluntary  Movement. 
ab  path  for  the  motor  nerves  of  the  trunk;  c,  fibers  of  the  facial  nerve;  B,  corpus 
callo'sum;  Nc,  nucleus  caudatus;  G,  i,  internal  capsule;  N.  I,  lenticular  nucleus;  P, 
pons;  N,f,  origin  of  the  facial;  Py,  pyramids  and  their  discussion;  01,  olive,  Gr, 
restif'orm  body;  PR,  posterior  root;  AR,  anterior  root;  x,  crossed,  and  z,  direct 
pyramidal  tracts.     {Landois.) 

Through  this  decussation  each  half  of  the  cerebrum  governs  the  muscle 
movements  of  the  opposite  side  of  the  body. 

The  fibers  composing  the  crossed  and  the  direct  pyramidal  tracts  are 


SPINAL   CORD.  173 

therefore  the  channels  by  which  the  volitional  nerve  impulses  are  conducted 
from  the  motor  area  of  the  cortex  to  the  multipolar  cells  in  the  ventral  cornua 
of  the  gray  matter  of  the  spinal  cord,  and  by  them  and  their  related  nerves 
transmitted  to  the  muscles. 

Spino-encephalic,  or  Sensor  Conduction. — The  nerve  impulses  that 
are  brought  to  the  spinal  cord  by  the  afferent  spinal  nerve-fibers  are  trans- 
mitted by  afferent  paths  in  the  cord  for  the  most  part  to  the  cortex  of  the 
cerebrum  where  they  are  translated  into  conscious  sensations.  These 
paths  are  therefore  termed  sensor.  The  sensor  tract  passes  through  the 
cord,  the  medulla  oblongata,  the  pons  Varolii,  the  superior  portion  of  the  crus 
cerebri,  the  posterior  third  of  the  posterior  limb  of  the  internal  capsule,  to 
sensor  perceptive  areas  in  the  cerebral  cortex.  The  sensor  pathway  decus- 
sates at  all  levels  of  the  spinal  cord  and  medulla,  and  therefore  the  sensibility 
of  each  side  of  the  body  is  associated  with  the  opposite  side  of  the  brain. 

The  paths  for  the  nerve  impulses  that  give  rise  to  different  sensations  have 
been  variously  located  by  different  observers.  The  pathway  for  the  im- 
pulses that  give  rise  to  the  sensations  of  temperature  has  been  located  in  the 
gray  matter;  the  pathway  for  the  impulses  that  give  rise  to  the  sensation  of 
pain  has  been  located  in  Gower's  tract;  the  pathway  for  tactile  impressions 
has  been  located  in  the  posterior  columns. 

Properties  of  the  Spinal  Cord. — Irritation  applied  directly  to  the  ventro- 
lateral white  columns  produces  muscular  movements,  but  no  pain;  they  are, 
therefore,  excitable,  but  insensible. 

The  surface  of  the  dorsal  columns  is  not  sensitive  to  direct  irritation, 
except  near  the  origin  of  the  dorsal  roots.  The  sensibility  is  due,  however, 
not  to  its  own  proper  fibers,  but  to  the  fibers  of  the  dorsal  root,  which  traverse 
it. 

Division  of  the  ventro-lateral  columns  abolishes  all  power  of  voluntary 
movement  in  the  lower  extremities. 

Division  of  the  dorsal  column  impairs  the  power  of  muscular  coordination, 
such  as  is  witnessed  in  locomotor  ataxia. 

The  gray  matter  is  probably  both  insensible  and  inexcitable  under  the 
influence  of  direct  stimulation. 

A  transverse  section  of  one  lateral  half  of  the  cord  produces — 

1.  On  the  same  side,  paralysis  of  voluntary  motion,  a  relative  or  absolute 
elevation  of  temperature,  and  an  increased  flow  of  blood  in  the  paralyzed 
parts;  hyperesthesia,  for  the  sense  of  contact,  tickling,  pain,  and  tempera- 
ture. 

2.  On  the  opposite  side,  complete  anesthesia  as  regards  contact,  tickling, 
and  temperature  in  the  parts  corresponding  to  those  which  are  paralyzed 


1 74  HUMAN  PHYSIOLOGY. 

in  the  opposite  side,  with  a  complete  preservation  of  voluntary  power 

and  of  the  muscular  sense. 

A  vertical  section  through  the  middle  of  the  gray  matter  results  in  the  loss 
of  sensation  on  both  sides  of  the  body  below  the  section,  but  no  loss  of  volun- 
tary power. 

Paralysis  from  Injuries  of  the  Spinal  Cord. 

Seat  of  Lesion. — If  it  be  in  the  lower  part  of  the  sacral  canal,  there  is  paralysis 
of  the  compressor  urethrae,  accelerator  urinae,  and  sphincter  ani  muscles;  no 
paralysis  of  the  muscles  of  the  leg. 

At  the  Upper  Limit  of  the  Sacral  Region. — Paralysis  of  the  muscles  of  the 
bladder,  rectum,  and  anus;  loss  of  sensation  and  motion  in  the  muscles  of  the 
legs,  except  those  supplied  by  the  anterior  crural  and  obturator — viz.,  psoas 
iliacus,  sartorius,  pectineus,  adductor  longus,  magnus,  and  brevis,  ob- 
turator, vastus  externus  and  internus,  etc. 

At  the  Upper  Limit  of  the  Lumbar  Region. — Sensation  and  motion  paralyzed 
in  both  legs;  loss  of  power  over  the  rectum  and  bladder;  paralysis  of  the  mus- 
cular walls  of  the  abdomen,  interfering  with  expiratory  movements. 

At  the  Lower  Portion  of  the  Cervical  Region. — Paralysis  of  the  legs,  etc., 
as  in  the  foregoing;  in  addition,  paralysis  of  all  the  intercostal  muscles  and 
consequent  interference  with  respiratory  movements;  paralysis  of  muscles 
of  the  upper  extremities,  except  those  of  the  shoulders. 

A  hove  the  Middle  of  the  Cervical  Region. — In  addition  to  the  preceding,  diffi- 
culty of  deglutition  and  vocalization,  contraction  of  the  pupils,  paralysis 
of  the  diaphragm,  scalene  muscles,  intercostals,  and  many  of  the  accessory 
respiratory  muscles;  death  resulting  immediately  from  arrest  of  respiratory 
movements. 

THE  MEDULLA  OBLONGATA. 

The  medulla  oblongata  is  the  expanded  portion  of  the  upper  part  of  the 
spinal  cord.  It  is  pyramidal  in  form  and  measures  i  \  inches  in  length, 
f  of  an  inch  in  breadth,  \  of  an  inch  in  thickness,  and  is  divided  into 
two  lateral  halves  by  the  anterior  and  posterior  median  fissures,  which  are 
continuous  with  those  of  the  cord.  Each  half  is  again  subdivided  by  minor 
grooves  into  four  columns — viz.,  ventral,  pyramid,  lateral  and  tract  olivary 
body,  restiform  body,  and  dorsal  pyramid. 

i.  The  ventral  pyramid  is  composed  partly  of  fibers  continuous  with  those 
of  the  ventral  column  of  the  spinal  cord,  but  mainly  of  fibers  derived  from 
the  lateral  tract  of  the  opposite  side  by  decussation.  The  united  fibers  then 
pass  upward  through  the  pons  Varolii  and  crura  cerebri,  and  for  the 
most  part  terminate  in  the  corpus  striatum  and  cerebrum. 


THE   MEDULLA   OBLONGATA. 


175 


2.  The  lateral  tract  is  continuous  with  the  lateral  columns  of  the  cord;  its 
fibers  in  passing  upward  take  three  directions — viz.,  an  external  bundle 
joins  the  restiform  body,  and  passes  into  the  cerebellum;  an  internal  bundle 
decussates  at  the  median  line  and  joins  the  opposite  ventral  pyramid;  a 
middle  bundle  ascends  beneath  the  olivary  body,  behind  the  pons,  to  the 
cerebrum,  as  the  fasciculus  teres.  The  olivary  body  of  each  side  is  an  oval 
mass,  situated  between  the  ventral  pyramid  and  restiform  body;  it  is  com- 
posed of  white  matter  externally  and  gray  matter  internally,  forming  the 
corpus  dentatum. 


Fig.  24. — View  of  Cerebellum  in  Section,  and  of  Fourth  Ventricle,  with 
the  Neighboring  Parts. — (Front  Sappey.) 
1.  Median  groove  fourth  ventricle,  ending  below  in  the  calamus  scriptorius,  with 
the  longitudinal  eminences  formed  by  the  fasciculi  teretes,  one^  on  each  side.  2. 
The  same  groove,  at  the  place  where  the  white  streaks  of  the  auditory  nerve  emerge 
from  it  to  cross  the  floor  of  the  ventricle.  3.  Inferior  peduncle  of  the  cerebellum, 
formed  by  the  restiform  body.  4.  Dorsal  pyramid;  above  this  is  the  calamus 
scriptorius.  s,  5.  Superior  peduncle  of  cerebellum,  or  processus  e  cerebello  ad  testes. 
6,  6.  Fillet  to  the  side  of  the  crura  cerebri. #  7,7.  Lateral  grooves  of  the  crura  cere- 
bri.    8.  Corpora  quadrigemina.     (After  Hirschfeld  and  Leveille.) 


The  restiform  body,  continuous  with  the  dorsal  column  of  the  cord,  also 
receives  fibers  from  the  lateral  column.  As  the  restiform  bodies  pass 
upward  they  diverge  and  form  a  space  (the  fourth  ventricle),  the  floor  of 
which  is  formed  by  gray  matter,  and  then  turn  backward  and  enter  the 
cerebellum. 

The  dorsal  pyramid  is  a  narrow  white  cord  bordering  the  posterior  median 
fissure;  it  is  continued  upward,  in  connection  with  the  fasciculus  teres,  to 
the  cerebrum. 


176  HUMAN  PHYSIOLOGY. 

The  gray  matter  of  the  medulla  is  continuous  with  that  of  the  cord.  It 
is  arranged  with  much  less  regularity,  becoming  blended  with  the  white 
matter  of  the  different  columns,  with  the  exception  of  the  anterior.  By  the 
separation  of  the  posterior  columns  the  transverse  commissure  is  exposed, 
forming  part  of  the  floor  of  the  fourth  ventricle;  special  collections  of  gray 
matter  are  found  in  the  posterior  portions  of  the  medulla,  connected  with  the 
roots  of  origin  of  different  cranial  nerves. 

Properties  and  Functions. — The  medulla  is  excitable  anteriorly,  and 
sensitive  posteriorly,  to  direct  irritation.     It  serves — 

1.  As  a  conductor  of  afferent  or  sensor  impulses  upward  from  the  cord, 
through  the  gray  matter  to  the  cerebrum. 

2.  As  a  conductor  of  efferent,  volitional  or  motor  impulses  from  the  brain  to 
the  spinal  cord  and  nerves,  through  its  ventral  pyramids. 

3.  As  a  conductor  of  coordinating  impulses  from  the  spinal  cord  to  the  cere- 
bellum, through  the  restiform  bodies. 

As  an  Independent  Reflex  Center. — The  medulla  oblongata  contains 
special  collections  of  gray  matter,  constituting  independent  nerve  centers 
presiding  over  different  functions,  some  of  which  are  as  follows — viz.: 

1.  A  center  which  controls  the  movements  of  mastication,  through  afferent 
and  efferent  nerves. 

2.  A  center  reflecting  impressions  which  influence  the  secretion  of  saliva. 

3.  A  center  for  sucking,  mastication,  and  deglutition,  whence  are  derived  motor 
stimuli  exciting  to  action  and  coordinating  the  muscles  of  the  palate, 
pharynx,  and  esophagus,  necessary  for  the  swallowing  of  the  food. 

4.  A  center  which  coordinates  the  muscles  concerned  in  the  act  of  vomiting. 

5.  A  speech  center,  coordinating  the  various  muscles  necessary  for  the  accom- 
pishment  of  articulation  through  the  hypoglossal,  facial  nerves,  and  the 
second  division  of  the  fifth  pair. 

6.  A  center  for  the  harmonization  of  muscles  concerned  in  expression,  reflect- 
ing its  impulses  through  the  facial  nerve. 

7.  A  cardiac  center,  which  exerts  (1)  an  accelerator  influence  over  the  heart's 
pulsations  through  nerve-fibers  emerging  from  the  thoracic  portion  of  the 
cord,  in  the  ventral  roots  of  the  second  and  third  thoracic  nerves  after  which 
they  pass  to  the  stellate  ganglion  around  the  cells  of  which  they  arborize, 
new  nerve  fibers  from  these  cells  then  pass  to  the  heart;  (2)  an  inhibitor 
or  retarding  influence  upon  the  action  of  the  heart,  through  fibers  of  the 
spinal  accessory  nerve  running  in  the  trunk  of  the  vagus.  The  cardio- 
inhibitor  center  is  in  a  state  of  tonic  activity  and  continuously  sends  im- 
pulses to  the  heart  which  exert  an  inhibitory  influence  upon  its  action.     It 


THE    MEDULLA   OBLONGATA.  1 77 

may  however  be  excited  or  inhibited  in  its  activity  by  nerve  impulses 
reflected  from  the  periphery  by  stimulation  of  various  afferent  nerves. 
The  heart  beat  will  in  consequence  be  slowed  or  increased  in  its  rate  by 
the  action  of  the  cardio-accelerator  center. 

8.  A  vaso-motor  center,  which,  by  alternately  contracting  and  dilating  the 
blood-vessels  through  nerves  distributed  in  their  walls,  regulates  the 
quantity  of  blood  distributed  to  an  organ  or  tissue,  and  thus  influences 
nutrition,  secretion,  and  calorification.  The  vaso-motor  center  is  situated 
in  the  medulla  oblongata  and  pons  Varolii,  between  the  corpora  quadri- 
gemina  and  the  calamus  scriptorius.  The  vaso-motor  fibers  having  their 
origin  in  this  center  descend  through  the  lateral  column  of  the  cord,  emerge 
through  the  ventral  roots  of  the  thoracic  or  upper  lumbar  nerves,  enter  the 
ganglia  of  the  sympathetic,  and  thence  pass  to  the  walls  of  the  blood-vessels, 
and  maintain  an  arterial  tonus;  they  may  be  divided  into  two  classes — viz., 
vaso-dilatators  and  vaso-constrictors. 

Division  of  the  cord  at  the  lower  border  of  the  medulla  is  followed  by  a 
dilatation  of  the  entire  vascular  system  and  a  marked  fall  of  the  blood  pres- 
sure. Electric  stimulation  of  the  distal  surface  of  the  cord  is  followed  by  a 
contraction  of  the  blood-vessels  and  a  rise  in  the  blood  pressure. 

The  vaso-motor  center  is  stimulated  directly  by  the  condition  of  the  blood 
in  the  medulla  oblongata.  When  the  blood  is  highly  venous  this  center  be- 
comes very  active,  the  blood-vessels  throughout  the  body  are  contracted,  and 
the  blood  current  becomes  swifter;  sudden  anemia  of  the  medulla  has  a  similar 
effect.  The  action  of  the  vasomotor  center  may  be  increased,  with  attend- 
ant rise  of  blood  pressure,  by  irritation  of  certain  afferent  nerve-fibers.  These 
are  known  as  pressor  fibers.  On  the  other  hand,  its  action  may  be  depressed 
by  other  fibers,  with  attendant  fall  of  blood  pressure.  These  are  known  as 
depressor  fibers. 

9.  A  diabetic  center,  irritation  of  which  causes  an  increase  in  the  amount  of 
urine  secreted  and  the  appearance  of  a  considerable  quantity  of  sugar  in 
the  urine. 

10.  Respiratory  center,  situated  near  the  origin  of  the  pneumogastric  nerves, 
presides  over  the  movements  of  respiration  and  its  modifications,  laughing, 
singing,  sobbing,  sneezing,  etc.  It  may  be  excited  refleccly  by  stimulation 
of  the  terminal  branches  of  the  vagus  nerve  during  an  act  of  expiration; 
or  automatically,  according  to  the  character  of  the  blood  circulating 
through  it;  an  excess  of  carbonic  acid  or  a  diminution  of  oxygen  increasing 
the  number  of  respiratory  movements;  a  reverse  condition  diminishing  the 
respiratory  movements. 

11.  A  spasm  center,  stimulation  of  which  gives  rise  to  convulsive  phenomena, 
such  as  coughing,  sneezing,  etc. 

12 


178  HUMAN  PHYSIOLOGY. 

12.  A  center  for  certain  ocular  functions,  governing  the  closure  of  the  eyelids 
and  dilatation  of  the  pupil. 

13.  A  sweat  center  is  also  localized  in  the  medulla. 

THE  PONS  VAROLII. 

The  pons  Varolii  is  united  with  the  cerebrum  above,  the  cerebellum  be- 
hind, and  the  medulla  oblongata  below.  It  consists  of  transverse  and  longi- 
tudinal fibers,  amidst  which  are  irregularly  scattered  collections  of  gray  or 
vesicular  nervous  matter. 

The  transverse  fibers  unite  the  two  lateral  halves  of  the  cerebellum. 

The  longitudinal  fibers  are  continuous — ■ 

1.  With  the  ventral  pyramids  of  the  medulla  oblongata,  which,  interlacing 
with  the  deep  layers  of  the  transverse  fibers,  ascend  to  the  crura  cerebri, 
forming  their  superficial  or  fasciculated  portions. 

2.  With  fibers  derived  from  the  olivary  fasciculus,  some  of  which  pass  to 
the  tubercula  quadrigemina,  while  others,  uniting  with  fibers  from  the  lat- 
eral and  posterior  columns  of  the  medulla,  ascend  in  the  deep  or  posterior 
portions  of  the  crura  cerebri. 

Properties  and  Functions. — The  superficial  portion  is  insensible  and 
inexcitable  to  direct  irritation;  the  deeper  portion  appears  to  be  excitable,  con- 
sisting of  descending  motor  fibers;  the  dorsal  portions  are  sensible,  but  in- 
excitable  to  irritation. 

Transmits  motor  and  sensor  impulses  from  and  to  the  cerebrum. 

The  gray  ganglionic  matter  consists  of  centers  which  convert  impressions 
into  more  or  less  conscious  sensations  and  originate  motor  impulses,  these 
taking  place  independent  of  any  intellectual  process;  they  are  the  seat  of 
instinctive  reflex  acts,  the  centers  which  assist  in  the  coordination  of  the 
automatic  movements  of  station  and  progression. 

THE  CRURA  CEREBRI. 

The  crura  cerebri  are  largely  composed  of  the  longitudinal  fibers  of  the 
pons  (anterior  pyramids,  fasciculi  teretes) ;  after  emerging  from  the  pons  they 
increase  in  size,  and  become  separated  into  two  portions  by  a  layer  of  dark- 
gray  matter,  the  locus  niger. 

The  superficial  portion,  the  crusta,  composed  of  the  anterior  pyramids, 
constitutes  the  motor  tract,  which  terminates,  for  the  most  part,  in  the  corpus 
striatum,  but  to  some  extent,  also,  in  the  cerebrum;  the  deep  portion,  made  up 
of  the  fasciculi  teretes  and  posterior  pyramids  and  accessory  fibers  from  the 


CORPORA   QUADRIGEMINA.  1 79 

cerebellum,  constitutes  the  sensor  tract  (the  tegmentum),  which  terminates  in 
the  optic  thalamus  and  cerebrum. 

Function. — The  crura  are  conductors  of  motor  and  sensor  impulses;  the 
gray  matter  assists  in  the  coordination  of  the  complicated  movements  of  the 
eyeball  and  iris,  through  the  motor  oculi  communis  nerve.  It  also  assists  in 
the  harmonization  of  the  general  muscular  movements,  as  section  of  one  crus 
gives  rise  to  peculiar  movements  of  rotation  and  somersaults  forward  and 
backward. 

THE  CORPORA  QUADRIGEMINA. 

The  corpora  quadigemina  are  four  small,  rounded  eminences,  two  on 
each  side  of  the  median  line,  situated  immediately  behind  the  third  ventricle, 
and  beneath  the  posterior  border  of  the  corpus  callosum. 

The  superior  tubercles  are  oblong  from  before  backward,  and  larger  than 
the  posterior,  which  are  hemispheric  in  shape;  they  are  grayish  in  color,  but 
consist  of  white  matter  externally  and  gray  matter  internally. 

Both  the  superior  and  posterior  tubercles  are  connected  with  the  optic 
thalami  by  commissural  bands,  named  the  superior  and  posterior  brachia, 
respectively.  They  receive  fibers  from  the  olivary  fasciculus  and  fibers  from 
the  cerebellum,  which  press  upward  to  enter  the  optic  thalami. 

The  corpora  geniculata  are  situated,  one  on  the  inner  side  and  one  on  the 
outer  side  of  each  optic  tract,  behind  and  beneath  the  optic  thalamus,  and 
from  their  position  are  named  the  corpora  geniculata  interna  and  externa;  the 
latter  is  a  terminal  for  some  of  the  true  visual  fibers. 

Functions. — The  corpora  quadrigemina  are  centers  associated  with  the  vis- 
ual centers.  Destruction  of  these  tubercules  is  immediately  followed  by  a  loss 
of  the  sense  of  sight;  moreover,  their  action  in  vision  is  crossed,  owing  to  the 
decussation  of  the  optic  tracts,  so  that  if  the  tubercle  of  the  right  side  be  de- 
stroyed by  disease  or  extirpated,  the  sight  is  lost  in  the  eye  of  the  opposite  side, 
and  the  iris  loses  its  mobility. 

The  tubercula  quadrigemina  as  nerve  centers  preside  over  the  reflex 
movements  which  cause  a  dilatation  or  contraction  of  the  iris,  irritation  of  the 
tubercles  causing  contraction,  destruction  causing  dilatation.  Removal  of  the 
tubercles  on  one  side  produces  a  temporary  loss  of  power  of  the  opposite  side 
of  the  body,  and  a  tendency  to  move  around  an  axis  is  manifested,  as  after  a 
section  of  one  crus  cerebri,  which,  however,  may  be  due  to  giddiness  and  loss 
of  sight. 

They  also  assist  in  the  coordination  of  the  complex  movements  of  the  eye, 
and  regulate  the  changes  of  the  iris  during  the  movements  of  accommodation. 


l8o  HUMAN  PHYSIOLOGY. 

CORPORA  STRIATA  AND  OPTIC  THALAMI. 

The  corpora  striata  are  two  large  ovoid  collections  of  gray  matter,  situated 
at  the  base  of  the  cerebrum,  the  larger  portions  of  which  are  embedded  in  the 
white  matter,  the  smaller  portions  projecting  into  the  anterior  part  of  the 
lateral  ventricle.  Each  striated  body  is  divided  by  a  narrow  band  of  white 
matter,  into  two  portions — viz.: 
i.  The  caudate  nucleus,  the  intraventricular  portion,  which  is  conic  in  shape, 

having  its  apex  directed  backward,  as  a  narrow,  tail-like  process. 
2.  The  lenticular  nucleus,  embedded  in  the  white  matter,  and  for  the  most 

part  external  to  the  ventricle.     On  the  outer  side  of  the  lenticular  nucleus 

is  found  a  narrow  band  of  white  matter,  the  external  capsule;  and  between 

it  and  the  convolutions  of  the  island  of  Reil,  a  thin  band  of  gray  matter, 

the  claustrum. 

The  corpora  striata  are  grayish  in  color,  and  when  divided,  present  trans- 
verse striations,  from  the  intermingling  of  white  fibers  and  gray  cells. 

The  optic  thalami  are  two  oblong  masses  situated  in  the  ventricles  poster- 
ior to  the  corpora  stiata,  and  resting  upon  the  posterior  portion  of  the  crura 
cerebri.  The  internal  surface,  projecting  into  the  lateral  ventricles,  is  white, 
but  the  interior  is  grayish,  rom  a  commingling  of  both  white  fibers  and  gray 
cells.  Separating  the  lenticular  nucleus  from  the  caudate  nucleus  and  the 
optic  thalamus  is  a  band  of  white  tissue,  the  internal  capsule. 

The  internal  capsule  is  a  narrow,  curved  tract  of  white  matter,  and  is,  for 
the  most  part,  an  expansion  of  the  motor  tract  of  the  crura  cerebri.  It  consists 
of  two  segments — an  anterior,  situated  between  the  caudate  nucleus  and  the 
anterior  surface  of  the  lenticular  nucleus,  and  a  posterior,  situated  between 
the  optic  thalamus  and  the  posterior  surface  of  the  lenticular  nucleus.  These 
two  segments  unite  at  an  obtuse  angle,  which  is  directed  toward  the 
median  line.  Pathologic  observation  has  shown  that  the  nerve-fibers  of  the 
direct  and  crossed  pyramidal  tracts  can  be  traced  upward  through  the  anterior 
two  thirds  of  the  posterior  segment  into  the  centrum  ovale,  where,  for  the 
most  part,  they  are  lost;  a  portion,  however,  remaining  united,  ascend  higher 
and  terminate  in  the  paracentral  lobule,  and  in  the  ascending  front  con- 
volution. Those  of  the  sensor  tract  can  be  traced  upward,  through  the  poster- 
ior third,  into  the  cerebrum,  where  they  probably  terminate  in  the  ascending 
parietal  and  the  superior  parietal  convolutions  and  in  the  gyrus  fornicatus. 

Functions. — The  corpora  striata  are  the  centers  in  which  terminate  some 
of  the  fibers  of  the  superficial  or  motor  tract  of  the  crura  cerebri;  others  pass 
upward  through  the  internal  capsule,  to  be  distributed  to  the  cerebrum. 
It  might  be  inferred,  from  their  anatomic  relations,  that  the  corpora  striata 


CEREBELLUM.  l8l 

are  motor  centers.  Irritation  by  a  weak  galvanic  current  produces  muscular 
movements  of  the  opposite  side  of  the  body;  destruction  of  their  substance  by 
a  hemorrhage,  as  in  apoplexy,  is  followed  by  a  paralysis  of  motion  of  the 
opposite  side  of  the  body,  but  there  is  no  loss  of  sensation.  When  the  hemor- 
rhagic destruction  involves  the  fibers  of  the  anterior  two  thirds  of  the  posterior 
segment  of  the  internal  capsule,  and  thus  separates  them  from  their  trophic 
centers  in  the  cortical  motor  region,  a  descending  degeneration  is  established, 
which  involves  the  direct  pyramidal  tract  of  the  same  side  and  the  crossed 
pyramidal  tract  of  the  opposite  side. 

Destruction  of  the  posterior  one  third  of  the  posterior  segment  of  the  inter- 
nal capsule  is  followed  by  a  loss  of  sensation  on  the  opposite  side  of  the  body 
and  a  loss  of  the  senses  of  smell  and  vision  on  the  same  side  (Charcot).  The 
precise  function  of  the  corpora  striata  is  unknown,  but  they  are  in  some 
way  connected  with  motion. 

The  optic  thalami  receive  the  fibers  of  the  tegmentum,  the  posterior  portion  of 
the  crura  cerebri.  They  are  insensible  and  inexcitable  to  direct  irritation. 
Removal  of  one  optic  thalamus,  or  destruction  of  its  substance  by  disease  or 
hemorrhage,  is  followed  by  a  loss  of  sensibility  of  the  opposite  side  of  the  body, 
but  there  is  no  loss  of  motion;  their  precise  function  is  also  unknown,  but  they 
are  in  some  way  connected  with  sensation.  In  both  cases  their  action  is 
crossed. 

THE  CEREBELLUM. 

The  cerebellum  is  situated  in  the  inferior  fossae  of  the  occipital  bone, 
beneath  the  posterior  lobes  of  the  cerebrum.  It  attains  its  maximum  weight, 
which  is  about  five  ounces,  between  the  twenty-fifth  and  fortieth  years,  the 
proportion  between  the  cerebellum  and  cerebrum  being  as  i  to  8f . 

It  is  composed  of  two  lateral  hemispheres  and  a  central  elongated  lobe,  the 
vermiform  process;  the  two  hemispheres  are  connected  with  each  other  by  the 
fibers  of  the  middle  peduncle,  forming  the  superficial  portion  of  the  pons  Varolii. 
The  cerebellum  is  brought  into  connection  with  the  medulla  oblongata  and 
spinal  cord  through  the  prolongation  of  the  restiform  bodies;  with  the  cere- 
brum, by  fibers  passing  upward  beneath  the  corpora  quadrigemina  and  the 
optic  thalami,  and  then  forming  part  of  the  diverging  cerebral  fibers. 

Structure. — It  is  composed  of  both  white  and  gray  matter,  the  former 
being  internal,  the  latter  external,  and  is  convoluted,  for  economy  of  space. 

The  white  matter  consists  of  a  central  stem,  the  interior  of  which  is  a  den- 
tated  capsule  of  gray  matter,  the  corpus  dentatum.  From  the  external  sur- 
face of  the  stem  of  white  matter  processes  are  given  off,  forming  the  lamina, 
which  are  covered  with  gray  matter. 


182  HUMAN  PHYSIOLOGY. 

The  gray  matter  is  convoluted  and  covers  externally  the  laminated  processes; 
a  vertical  section  through  the  gray  matter  reveals  the  following  structures: 
i.  A  delicate  connective-tissue  layer,  just  beneath  the  pia  mater,  containing 

rounded  corpuscles,  and  with  branching  fibers  passing  toward  the  external 

surface. 

2.  The  cells  of  Pur  kin  je,  forming  a  layer  of  large,  nucleated,  branched  nerve- 
cells  sending  off  processes  to  the  external  layer. 

3 .  A  granular  layer  of  small  but  numerous  corpuscles. 

4.  A  nerve-fiber  layer,  formed  by  a  portion  of  the  white  matter. 

Properties  and  Functions. — Irritation  of  the  cerebellum  is  not  followed 
by  any  evidences  either  of  pain  or  convulsive  movements;  it  is,  therefore, 

insensible  and  inexcitable. 

Coordination  of  Movements. — Removal  of  the  superficial  portions  of  the 
cerebellum  in  pigeons  ^products  feebleness  and  want  of  harmony  in  the  muscular 
movements;  as  successive  slices  are  removed,  the  movements  become  more 
irregular,  and  the  pigeon  becomes  restless;  when  the  last  portions  are  removed, 
all  powers  of  flying,  walking,  standing,  etc.,  is  entirely  gone,  and  the  equili- 
brium can  not  be  maintained,  the  power  of  coordinating  muscular  movements 
being  wholly  lost.  The  same  results  have  been  obtained  by  operating  on  all 
classes  of  animals. 

The  following  symptoms  were  noticed  by  Wagner,  after  removing  the 
whole  or  a  large  part  of  the  cerebellum: 

1.  A  tendency  on  the  part  of  the  animal  to  throw  itself  on  one  side,  and  to 
extend  the  legs  as  far  as  possible. 

2.  Torsion  of  the  head  on  the  neck. 

3.  Trembling  of  the  muscles  of  the  body,  which  was  general. 

4.  Vomiting  and  occasional  liquid  evacuations. 

Forced  Movements. — Division  of  one  cms  cerebelli  causes  the  animal  to 
fall  on  one  side  and  roll  rapidly  on  its  longitudinal  axis.  According  to  Schiff, 
if  the  peduncle  be  divided  from  behind,  the  animal  falls  on  the  same  side  as 
the  injury;  if  the  section  be  made  in  front,  the  animal  turns  to  the  opposite 
side. 

Disease  of  the  cerebellum  partially  corroborates  the  result  of  experiments; 
in  many  cases  symptoms  of  unsteadiness  of  gait,  from  a.  want  of  coordination 
have  been  noticed. 

Comparative  anatomy  reveals  a  remarkable  correspondence  between  the 
development  of  the  cerebellum  and  the  increase  in  complexity  of  muscular 
actions.  It  attains  a  much  greater  development,  relatively  to  the  rest  of  the 
brain,  in  those  animals  whose  movements  are  very  complex  and  varied  in 
character,  such  as  the  kangaroo,  shark,  and  swallow. 


CEREBRUM.  183 

The  cerebellum  may  possibly  exert  some  influence  over  the  sexual  functions, 
but  physiologic  and  pathologic  facts  are  opposed  to  the  idea  of  its  being  the 
seat  of  the  sexual  instinct.  It  appears  to  be  simply  a  center  for  the  coordina- 
tion and  equilibration  of  muscular  movements. 

THE  CEREBRUM. 

The  cerebrum  is  the  largest  portion  of  the  encephalic  mass,  constituting 
about  four  fifths  of  its  weight;  the  average  weight  of  the  adult  male  brain  is 
from  forty-eight  to  fifty  ounces,  or  about  three  pounds  while  that  of  the  adult 
female  is  about  five  ounces  less.  After  the  age  of  forty  the  weight  of  the  cere- 
brum gradually  diminishes  at  the  rate  of  one  ounce  every  ten  years.  In 
idiots  the  brain  weight  is  often  below  the  normal,  at  times  not  amounting  to 
more  than  twenty  ounces. 

The  cerebrum  is  connected  with  the  pons  Varolii  and  medulla  oblongata 
through  the  crura  cerebri,  and  with  the  cerebellum  through  the  superior 
peduncles.  It  is  divided  into  two  lateral  halves,  or  hemispheres,  by  the 
longitudinal  fissure  running  from  before  backward  in  the  median  line;  each 
hemisphere  is  composed  of  both  white  and  gray  matter,  the  former  being 
internal,  the  latter  external;  it  covers  the  surfaces  of  the  hemisphere  which  are 
infolded,  forming  fissures  and  convolutions. 

Fissures. 

1.  The  fissure  of  Sylvius  is  one  of  the  most  important;  it  is  the  first  to  appear 
in  the  development  of  the  fetal  brain,  being  visible  at  about  the  third  month; 
in  the  adult  it  is  quite  deep  and  well  marked,  running  from  the  under  sur- 
face of  the  brain  upward,  outward,  and  backward,  and  forms  a  boundary 
between  the  frontal  and  temporosphenoid  lobes. 

2.  The  fissure  of  Rolando  is  second  in  importance,  and  runs  from  a  point  on 
the  convexity  near  the  median  line  transversely  outward  and  downward 
toward  the  fissure  of  Sylvius,  but  does  not  enter  it.  It  separates  the  frontal 
from  the  parietal  lobe. 

3.  The  parietal  fissure,  arising  a  short  distance  behind  the  fissure  of  Rolando, 
upon  the  convexity  of  the  hemisphere,  runs  downward  and  backward  to  its 
posterior  extremity. 

4.  The  parieto-occipital  fissure  separates  the  occipital  from  the  parietal  lobe. 
Beginning  upon  the  outer  surface  of  the  cerebrum,  it  is  continued  on  the 
mesial  aspect  downward  and  forward  until  it  terminates  in  the  calcarine 
fissure. 

5.  The  callosomarginal  fissure  lies  upon  the  mesial  surface,  where  it  runs 
parallel  with  the  corpus  callosum. 


1 84 


HUMAN  PHYSIOLOGY. 


Secondary  fissures  of  importance  are  found  in  different  lobes  of  the  cere- 
brum, separating  the  various  convolutions.  In  the  anterior  lobe  are  found  the 
precentral,  superior  frontal,  and  inferior  frontal  fissures;  in  the  temporosphe- 
noid  lobes  are  found  the  first  and  second  temporosphenoid  fissure;  in  the  occipi- 
tal lobe,  the  calcarine  and  hippocampal  fissures. 

Convolutions.    Frontal  Lobe. 

The  ascending  frontal  or  precentral  convolution,  situated  in  front  of  the  fissure 
of  Rolando  runs  downward  and  forward;  it  is  continuous  above  with  the 
anterior  frontal,  and  below  with  the  inferior  frontal,  convolution. 


Fig.  25. — Diagram  showing  Fissures  and  Convolutions  on  the  Lateral 
Aspect  of  the  Left  Hemi-Cerebrum. 
F.  Frontal.  P.  Parietal.  T.  Temporal  and  O.  Occipital  lobes.  S.  Fissure  of 
Sylvius.  EP  S.  Epi-sylvian.  PRS.  Pre-sylvian.  S  B  S.  Sub-sylvian  fissures.  C. 
Central  fissure  or  Fissure  of  Rolando.  PRO  Pre-central  fissure.  S  P  F  R.  Super- 
frontal  fissure.  M  E  F  R.  Medi-frontal  fissure.  S  B  F  R.  Sub-frontal  fissure.  P  C.  P  C. 
Post-central  fissure.  PTL.  Parietal  fissure.  PA  ROC.  Par-occipital,  EXOCC. 
Ex-occipital  fissures.  SPTMP.  Super-temporal  fissure.  MTMP.  Medi-temporal 
fissure. 

The  superior  frontal  convolution  is  bounded  internally  by  the  longitudinal 
fissure,  and  externally  by  the  superior  frontal  fissure;  it  is  connected  with  the 
superior  end  of  the  frontal  convolution,  and  runs  downward  and  forward  to 
the  anterior  extremity  of  the  frontal  lobe,  where  it  turns  backward,  and  rests 
upon  the  orbital  plate  of  the  frontal  bone. 

The  middle  frontal  convolution,  the  largest  of  the  three,  runs  from  behind 
forward,  along  the  sides  of  the  lobe,  to  its  anterior  part;  it  is  bounded  above 
by  the  superior  and  below  by  the  inferior  frontal  fissures. 

The  inferior  frontal  convolution  winds  around  the  ascending  branch  of  the 
fissure  of  Sylvius,  in  the  anterior  and  inferior  portion  of  the  cerebrum. 


CEREBRI"  M. 


i»S 


Parietal  Lobe. — The  ascending  parietal  convolution  is  situated  just  behind 
the  fissure  of  Rolando,  running  downward  and  forward;  above,  it  becomes 
continuous  with  the  upper  parietal  convolution,  and  below,  winds  around  to 
be  united  with  the  ascending  frontal. 

The  upper  parietal  convolution  is  situated  between  the  parietal  and  longitu- 
dinal fissures. 

The  supramarginal  convolution  winds  around  the  superior  extremity  of  the 
fissure  of  Sylvius. 

The  angular  convolution,  a  continuation  of  the  preceding,  follows  the  parie- 
tal fissure  to  its  posterior  extremity,  and  then  makes  a  sharp  angle  downward 
and  forward. 


Fig.   26. — Diagram    showing   Fissures   and   Convolutions   on  the  Mesal  As- 
pect of  the  Left  Hemi-cerebrum. 
C  Upper  extremity  of  the  central  fissure.    PARC.  Para-central  fissure.  SPCL. 
Super-callosal  fissure.    C  L.  Callosal  fissure.     O  C.  Occipital  fissure.     C  L  C.  Calca- 
rine  fissure.    CLT.  Collateral  fissure. 


Temporosphenoid  Lobe. — Contains  three  well-marked  convolutions,  the 
superior,  middle  and  inferior,  separated  by  well-defined  fissures,  and  continu- 
ous posteriorly  with  the  convolutions  of  the  parietal  lobe. 

The  occipital  lobe  lies  behind  the  parietooccipital  fissure,  and  contains 
the  superior,  middle  and  inferior  convolutions,  not  well  marked. 

The  central  lobe,  or  island  of  Reil,  situated  at  the  bifurcation  of  the  fissure 
of  Sylvius,  is  a  triangular-shaped  cluster  of  six  convolutions,  the  gyri  operti, 
which  are  connected  with  those  of  the  frontal,  parietal,  and  temporosphenoid 
lobes. 

Upon  the  inner  or  mesial  aspect  of  the  hemisphere  are  found  (Fig.  25) — 


1 86  HUMAN  PHYSIOLOGY. 

i.  The  paracentral  convolution,  lying  in  the  region  of  the  upper  extremity 
of  the  fissure  of  Rolando;  it  contains  the  large  giant  cells  of  Betz.  Injury 
to  this  convolution  is  followed  by  degeneration  of  the  motor  tract. 

2.  The  callosal  convolution,  lying  below  the  supercallosal  fissure.  Running 
parallel  with  the  corpus  callosum,  it  terminates  at  its  posterior  border  in  the 
hippocampal  gyrus. 

3.  The  gyrus  hippocampus  is  formed  by  the  union  of  the  preceding  con- 
volution with  the  occipitotemporal.  It  runs  forward  and  terminates  in  a 
hooked  extremity — uncus. 

4.  The  quadrate  lobule,  or  precuneus,  lies  between  the  upper  extremity  of  the 
callosomarginal  fissure  and  the  parieto-occipital. 

5.  The  cuneus  lies  posteriorly  to  the  quadrate  lobule.  It  is  a  wedge-shaped 
mass  inclosed  by  the  calcarine  and  occipital  fissures. 

Structure. — The  gray  matter  of  the  cerebrum,  about  $•  of  an  inch  thick, 
is  composed  of  five  layers  of  nerve-cells: 

1.  A  superficial  layer,  containing  a  few  small  multipolar  ganglion  cells. 

2.  Small  ganglion  cells,  pyramidal  in  shape. 

3.  A  layer  of  large  pyramidal  ganglion  cells  with  processes  running  off  super- 
iorly and  laterally. 

4.  The  granular  formation  containing  nerve-cells. 

5.  Spindle-shaped  and  branching  nerve-cells  of  a  moderate  size. 
The  white  matter  consists  of  three  distinct  sets  of  fibers. 

1.  The  diverging  or  peduncular  fibers,  are  mainly  derived  from  the  columns  of 
the  cord  and  medulla  oblongata;  passing  upward  through  the  crura  cerebri 
they  receive  accessory  fibers  from  the  olivary  fasciculus,  corpora  quadri- 
gemina,  and  cerebellum.  Some  of  the  fibers  terminate  in  the  optic  thalami 
and  corpora  striata,  while  others  radiate  into  the  anterior,  middle  and 
posterior  lobes  of  the  cerebrum. 

2.  The  trasverse  commissural  fibers  connect  the  two  hemispheres,  through  the 
corpus  callosum  and  anterior  and  posterior  commissures. 

3.  The  longitudinal  commissural  fibers  connect  different  parts  of  the  same 
hemisphere. 

Functions. — The  cerebral  hemispheres  are  the  centers  of  the  nervous 
system  through  which  are  manifested  all  the  phenomena  of  the  mind;  they 
are  the  centers  in  which  impressions  are  registered  and  reproduced  subse- 
quently as  ideas;  they  are  the  seat  of  intelligence,  reason,  and  will. 

However  important  a  center  the  cerebrum  may  be  for  the  exhibition  of  this 
highest  form  of  nervous  action,  it  is  not  directly  essential  for  the  continuance 
of  life,  for  it  does  not  exert  any  control  over  those  automatic  reflex  acts,  such 
as  respiraton,  circulation,  etc.,  which  regulate  the  functions  of  organic  life. 


CEREBRUM.  187 

From  the  study  of  comparative  anatomy,  pathology,  vivisection,  etc.,  evi- 
dence has  been  obtained  which  throws  some  light  upon  the  physiology  of  the 
cerebral  hemispheres. 

1.  Comparative  anatomy  shows  that  there  is  a  general  connection  between  the 
size  of  the  brain,  its  texture,  the  depth  and  number  of  convolutions,  and  the 
exhibition  of  mental  power.  Throughout  the  entire  animal  series,  the 
increase  in  intelligence  goes  hand  in  hand  with  an  increase  in  the  develop- 
ment of  the  brain.  In  man  there  is  an  enormous  increase  in  size 
over  that  of  the  highest  animals,  the  anthropoids.  The  most  cultivated 
races  of  men  have  the  greatest  cranial  capacity;  that  of  the  educated 
European  being  about  116  cubic  inches,  that  of  the  Australian  being  about 
60  cubic  inches,  a  difference  of  56  cubic  inches.  Men  distinguished  for 
great  mental  power  usually  have  large  and  well-developed  brains;  that  of 
Cuvier  weighed  64  ounces;  that  of  Abercrombie,  63  ounces;  the  average 
being  about  48  to  50  ounces.  Not  only  the  size,  but,  above  all,  the 
texture,  of  the  brain  must  be  taken  into  consideration. 

2.  Pathology. — Any  severe  injury  or  disease  disorganizing  the  hemispheres 
is  at  once  attended  by  a  disturbance  or  an  entire  suspension  of  mental 
activity.  A  blow  on  the  head,  producing  concussion,  or  undue  pressure 
from  cerebral  hemorrhage,  destroys  consciousness;  physical  and  chemic 
alterations  in  the  gray  matter  have  been  shown  to  coexist  with  insanity, 
and  with  loss  of  memory,  speech,  etc.  Congenital  defects  of  organization 
from  imperfect  development  are  usually  accompanied  by  a  corresponding 
deficiency  of  intellectual  power  and  of  the  higher  instincts.  Under  these 
circumstances  no  great  advance  in  mental  development  can  be  possible,  and 
the  intelligence  remains  of  a  low-grade.  In  congenital  idiocy  not  only  is 
the  brain  of  small  size,  but  it  is  wanting  in  proper  chemic  composition, 
phosphorus,  a  characteristic  ingredient  of  the  nervous  tissue,  being  largely 
diminished  in  amount. 

3.  Experimentation  upon  the  lower  animals — e.  g.,  the  removal  of  the  cerebral 
hemispheres,  is  attended  by  results  similar  to  those  observed  in  disease  and 
injury.  Removal  of  the  cerebrum  in  pigeons  produces  complete  abolition 
of  intelligence,  and  destroys  the  capability  of  performing  spontaneous 
movements.  The  pigeon  remains  in  a  condition  of  profound  stupor, 
which  is  not  accompanied,  however,  by  a  loss  of  sensation  or  of  the  power 
of  producing  reflex  or  instinctive  movements.  The  pigeon  can  be  tem- 
porarily aroused  by  pinching  the  feet,  loud  noises,  lights  placed  before  the 
eyes,  etc.,  but  soon  relapses  into  a  state  of  quietude,  being  unable  to  remem- 
ber impressions  and  connect  them  with  any  train  of  ideas,  the  faculties 
of  memory,  reason,  and  judgment  being  completely  abolished. 


1 88  HUMAN  PHYSIOLOGY. 

CEREBRAL  LOCALIZATION  OF  FUNCTIONS. 

From  experiments  made  upon  animals,  and  from  the  results  of  clinical 
and  post-mortem  observations  upon  men,  it  has  been  shown  that  the  phen- 
omena of  organic  and  psychic  life  are  presided  over  by  anatomically  localized 
centers  in  the  brain.  A  knowledge  of  the  position  of  these  centers  becomes  of 
the  highest  importance  in  localizing  the  seat  of  lesions,  thrombi,  hemorrhages, 
new  growths,  etc.,  which  show  themselves  in  paralyses,  epilepsies,  etc.  It 
has  not  been  possible  to  thus  localize  all  functions,  and  to  many  parts  of  the 
brain  no  special  use  can  be  assigned.  The  following  are  the  centers  most 
definitely  mapped  out  and  that  are  of  paramount  importance. 

Motor  Centers. — These  are  in  the  cortical  gray  matter,  and  are  arranged 
along  either  side  of  the  fissure  of  Rolando.  This  area  is  known  as  the  motor 
area  or  motor  zone,  stimulation  of  which  is  followed  by  convulsive  movements 
of  the  muscles  of  the  opposite  side  of  the  body,  while  destruction  of  the  gray 
matter  of  this  area  is  followed  by  permanent  paralysis  of  the  muscles  of  the 
opposite  side.  From  experiments  made  upon  monkeys,  Ferrier  has  mapped 
out  a  number  of  motor  centers.  In  a  general  way  it  may  be  said  that  the  upper 
third  of  the  ascending  frontal  and  parietal  convolutions  about  this  fissure 
preside  over  the  movements  of  the  leg  of  the  opposite  side  of  the  body;  the 
middle  third  controls  the  movements  of  the  arm;  the  upper  part  of  the  inferior 
third  is  the  facial  area.  The  lowest  part  of  the  inferior  third  governs  the 
motility  of  the  lips  and  tongue,  and  this  space,  with  the  posterior  extremity 
of  the  third  frontal  convolution,  constitutes  the  speech  center. 

The  experiments  of  Horsley  and  Schafer  have  enabled  them  to  furnish 
a  new  diagrammatic  representation  of  the  motor  area  and  more  accurately 
to  define  the  special  areas  upon  the  lateral  and  mesial  aspects  of  the  brain  of 
the  monkey.  The  boundaries  of  the  general  and  special  areas,  as  determined 
by  these  observers,  will  be  readily  understood  by  an  examination  of  Figures 
27   and   28. 

For  diagnostic  purposes  the  motor  areas  for  the  face  and  limbs  have  been 
subdivided  as  follows: 

1.  The  face  area  may  be  divided  into  an  upper  part,  comprising  about  one 
third,  and  a  lower  part,  comprising  the  remaining  two  thirds.  In  the 
upper  part  are  centers  governing  the  movements  of  the  muscles  of  the 
opposite  angle  of  the  mouth  and  of  the  lower  face.  The  anterior  portion 
of  the  lower  two  thirds  controls  the  movements  of  the  vocal  cords,  and 
may  be  regarded  as  a  laryngeal  center;  the  posterior  portion  governs  the 
opening  and  shutting  of  the  mouth  and  the  protrusion  and  retraction  of 
the  tongue. 


CEREBRAL    LOCALIZATION    OF    FUNCTIONS. 


189 


The  upper  limb  area  may  be  subdivided  as  follows:  The  upper  part 
controls  the  movements  of  the  shoulder;  posteriorly  and  below  this  point 
are  centers  for  the  elbow;  below  and  anteriorly,  centers  for  the  wrist  and 
finger  movements,  while  lowest  and  posteriorly,  centers  governing  the 
thumb. 


Fig.  27. — Diagram  of  the  Motor  Areas  on  the  Outer  Surface  of  a  Monkey's 
Brain. — (Horsley  and  Schdfer.) 


Fig.  28.- 


-DlAGRAM   OF   THE  MOTOR  AREAS   ON  THE  MARGINAL  CONVOLUTION   OF   A 

Monkey's  Brain. — (Horsley  and  Schdfer.) 


The  leg  area  may  be  subdivided  as  follows:  The  anterior  part,  both 
on  the  mesial  and  lateral  surfaces,  contains  centers  governing  the  hip  and 
thigh  movements;  in  the  posterior  part  are  centers  for  the  movements  of 
the  leg  and  toes.  The  center  for  the  great  toe  has  been  located  in  the 
paracentral  lobule. 


190 


HUMAN  PHYSIOLOGY. 


4.  The  trunk  area,  situated  largely  on  the  mesial  surface,  contains  anteriorly 
centers  governing  the  rotation  and  arching  of  the  spine,  while  posteriorly 
are  found  centers  governing  movements  of  the  tail  and  pelvis. 

5.  The  head  area,  or  area  for  visual  direction,  contains  centers  excitation  of 
which  causes  "opening  of  the  eyes,  dilatation  of  the  pupils,  and  turning 
of  the  head  to  the  opposite  side,  with  conjugate  deviation  of  the  eyes  to 
that  side." 

From  experiments  made  on  animals  corrected  by  observations  of  lesions 
of  the  human  brain  corresponding  centers  have  been  located  in  man  as 
shown  in  Fig.  29  and  30. 


&»% 


CONCRETE   CONCEPT 


Fig.  29.— The  Areas  and  Centers  of  the  Lateral  Aspect  of  the  Human 
Hemi-cerebrum. — (C.  K.  Mills.) 

Center  for  Speech. — Pathologic  investigations  have  demonstrated  that 
the  left  third  frontal  convolution  is  of  essential  importance  for  speech. 
Adjoining  this  convolution  are  the  centers  controlling  the  motility  of  the  lips, 
tongue,  etc.  In  the  majority  of  cases  the  speech  centers  are  on  the  left  side 
of  the  brain,  though  in  exceptional  cases  they  are  on  the  right  side,  especially 
in  left-handed  persons.  In  deaf  mutes  this  convolution  is  very  imperfectly 
developed,  while  in  monkeys  it  is  quite  rudimentary. 


CEREBRAL    LOCALIZATION    OF    FUNCTIONS. 


I9I 


Lesions  of  the  third  frontal  convolution  on  the  left  side,  if  the  patient  be 
right-handed,  produce  the  various  forms  of  aphasia,  or  the  partial  or  complete 
loss  of  the  power  of  articulate  speech. 

Aphasia  is  of  many  degrees  and  kinds.  In  ataxic  aphasia  the  patient  is 
unable  to  communicate  his  thoughts  by  words,  there  being  an  inability  to 
execute  the  movements  of  the  mouth,  etc.,  necessary  for  speech.  In  agraphia 
aphasia  there  is  an  inability  to  execute  the  movements  necessary  for  writing, 
though  the  mental  processes  are  retained.  In  the  ataxic  form  the  lesion  is 
in  the  third  frontal  convolution,  and  in  the  agraphic  form  it  is  in  the  arm 
center. 


Fig.  30.— The  Areas  and  Centers  of  the  Mesial  Aspect  of  the  Human 
Hemi-cerebrum.— (C.  K.  Mills.) 


In  amnesic  aphasia  there  is  a  loss  of  the  memory  of  words,  the  purest  exam- 
ples of  which  consist  of  the  affections  known  as  word-deafness  and  word- 
blindness.  In  word-deafness  the  patient  can  not  understand  vocal  speech, 
though  he  is  capable  of  hearing  other  sounds.  This  condition  is  associated 
with  lesion  of  the  first  temporal  convolution.  In  word-blindness  the  patient 
can  not  name  a  letter  or  a  word  printed  or  written,  though  he  can  see  all 
other  objects.  This  condition  is  associated  with  impairment  of  the  visual 
centers. 


192 


HUMAN  PHYSIOLOGY. 


Figure  3 1  will  illustrate  the  conditions  in  the  various  forms  of  aphasia. 
Impressions  are  constantly  passing  from  eye  and  ear  to  the  visual  and  audi- 
tory centers  and  are  there  being  registered.  Commissural  fibers  connect 
these  centers  with  the  arm  and  speech  centers,  which  in  turn  are  connected 
by  efferent  fibers  with  the  muscles  of  the  hand  and  of  the  vocal  apparatus. 

Muscular  movements  of  the 
eye,  hand,  and  mouth  are  also 
registered  by  means  of  the 
efferent  fibers,  s,  s',  s". 

Sensor  Centers. — These  are 
the  centers  in  which  the  affer- 
ent impulses  are  translated 
into  conscious  sensations.  The 
most  important  are: 

The  visual  center,  located  in 
the  occipital  lobe  and  especially 
in  the  cuneus.  Unilateral  de- 
struction of  this  area  results  in 
hemianopsia,  or  blindness  of 
the  corresponding  halves  of  the 
two  retinae.  Destruction  of 
both  occipital  lobes  in  man 
results  in  total  blindness. 
Stimulation  or  irritation  of  the 
visual  center  causes  photopsia, 
or  hallucinations  of  sight,  in 
corresponding  halves  of  the 
retinas.  There  have  been  in- 
stances of  injury  of  these  parts 
when  sensations  of  color  were 
abolished  with  preservation  of 
those  of  space  and  light,  thus 
showing  a  special  localization  of  the  color  center.  Recent  experiments 
show  that  the  centers  of  the  two  hemispheres  are  united,  as  ocular 
fatigue  of  an  unused  eye  was  found  to  be  proportional  to  the  fatigue  of  the 
exercised  one. 

The  auditory  centers  are  located  in  the  temporosphenoid  lobes.  Word- 
deafness  is  associated  with  softening  of  these  parts,  and  their  complete 
removal  results  in  deafness. 

The  gustatory  and  olfactory  centers  are  located  in  the  uncinate  gyrus,  on  the 


Fig.  31. 


SYMPATHETIC    NERVE    SYSTEM.  1 93 

inner  side  of  the  temporosphenoid  lobes.  There  does  not  seem  to  be  any 
differentiation,  up  to  this  time,  of  these  two  centers. 

The  center  for  tactile  impressions  was  located  by  Ferrier  in  the  hippocampal 
region.  Horsley  and  Schafer  found  that  destructive  lesions  of  the  gyrus 
fornicatus  were  followed  by  hemianesthesia  of  the  opposite  side  of  the  body, 
which  was  more  or  less  marked  and  persistent.  These  observers  conclude 
that  the  limbic  lobe  "is  largely,  if  not  exclusively,  concerned  in  the  apprecia- 
tion of  sensations,  painful  and  tactile." 

The  superior  and  middle  frontal  convolutions  appear  to  be  the  seats  of  the 
reason,  intelligence,  and  will.  Destruction  of  these  parts  is  followed  by 
proportional  hebetude,  without  any  impairment  of  sensation  or  motion. 

THE  SYMPATHETIC  NERVE  SYSTEM. 

The  sympathetic  nerve  system  consists  of  a  chain  of  ganglia  connected 
by  longitudinal  nerve  filaments,  situated  on  each  side  of  the  spinal  column, 
running  from  above  downward. 

The  chain  of  ganglia  is  divided  into  groups,  and  named  according  to  the 
location  in  which  they  are  found — viz.,  cranial,  four  in  number;  cervical, 
three;  thoracic,  twelve;  lumbar,  five;  sacral,  five;  coccygeal,  one.  Each  gan- 
glion consists  of  a  collection  of  vesicular  nervous  matter,  bundles  of  non-medul- 
lated  nerve-fibers,  embedded  in  a  capsule  of  connective  tissue.  The  ganglia 
are  in  connection  with  motor  fibers  from  the  cerebro-spinal  nervous  system. 

The  ganglia  give  origin  to  nerve-fibers,  from  which  branches  are  distrib- 
uted to  the  glands,  arteries,  and  non-striated  muscles,  in  the  walls  of  viscera. 

Cephalic  Ganglia. 

i .  The  ophthalmic  or  ciliary  ganglion  is  situated  in  the  orbital  cavity,  posterior 
to  the  eyeball;  it  is  of  small  size  and  of  a  reddish-gray  color;  receives  fila- 
ments of  communication  from  the  motor  oculi  ophthalmic  branch  of  the 
fifth  pair,  and  the  carotid  plexus.  Its  filaments  of  distribution  are  the 
ciliary  nerves,  which  consist  of — 

(a)  Motor  fibers  for  the  circular  fibers  of  the  iris  and  ciliary  muscle. 

(b)  Sensor  fibers  for  the  cornea,  iris,  and  associated  parts. 

(c)  Vaso-motor  fibers  for  the  blood-vessels  of  the  choroid,  iris,  and  retina. 

(d)  Motor  fibers  for  the  dilator  fibers  of  the  iris. 

2.  The  spheno-palatine  or  Meckel's  ganglion,  triangular  in  shape,  is  situated 
in  the  sphenomaxillary  fossa;  receives  filaments  from  the  facial  (Vidian 
nerve)  and  the  superior  maxillary  branch  of  the  fifth  nerve.  Its  filaments 
of  distribution  pass  to  the  gums,  the  soft  palate  and  associated  parts. 

3.  The  otic  or  Arnold's  ganglion  is  of  small  size,  oval  in  shape,  and  situated 

J3 


194  HUMAN  PHYSIOLOGY. 

beneath  the  foramen  ovale;  receives  a  motor  filament  from  the  facial  and 
sensor  filaments  from  the  glossopharyngeal  and  fifth  nerves;  sends  filaments 
to  the  mucous  membrane  of  the  tympanic  cavity  and  to  the  tensor  tympani 
muscle. 
4.  The  submaxillary  ganglion,  situated  in  the  submaxillary  gland,  receives 
filaments  from  the  chorda  tympani,  sensor  filaments  from  the  lingual 
branch  of  the  fifth  nerve,  and  filaments  from  the  sympathetic.  The  chorda 
tympani  nerve  supplies  vaso-dilatator  and  secretor  fibers  to  the  submaxillary 
and  sublingual  glands.  The  fifth  nerve  endows  the  glands  with  sensibility, 
while  the  sympathetic  supplies  vaso-constrictor  fibers  to  the  blood-vessels  of 
the  glands. 

Cervical  Ganglia. 

The  superior  cervical  ganglion  is  fusiform  in  shape,  of  a  grayish-red  color, 
and  situated  opposite  the  second  and  third  cervical  vertebrae;  it  sends  branches 
to  form  the  carotid  and  cavernous  plexuses,  which  branches  follow  the  course 
of  the  carotid  arteries  to  their  distribution;  also  sends  branches  to  join  the 
glossopharyngeal  and  pneumogastric,  to  form  the  pharyngeal  plexus. 

The  middle  cervical  ganglion,  the  smallest  of  the  three,  is  occasionally 
absent;  it  is  situated  opposite  the  fifth  cervical  vertebra;  sends  branches  to 
the  superior  and  inferior  cervical  ganglia  and  to  the  thyroid  artery. 

The  inferior  cervical  ganglion,  irregular  in  form,  is  situated  opposite  the 
last  cervical  vertebra;  it  is  frequently  united  with  the  first  thoracic  ganglion. 

The  superior,  middle,  and  inferior  cardiac  nerves,  arising  from  these  cervical 
ganglia,  pass  downward  and  forward  to  form  the  deep  and  superficial  cardiac 
plexuses  located  at  the  bifurcation  of  the  trachea,  from  which  branches  are 
distributed  to  the  heart,  coronary  arteries,  etc. 

The  thoracic  ganglia  are  usually  twelve  in  number,  and  are  placed  against 
the  heads  of  the  ribs  behind  the  pleura;  they  are  small  in  size  and  gray  in 
color;  they  communicate  with  the  cerebrospinal  nerves  by  two  filaments,  one 
of  which  is  white,  the  other  gray. 

The  great  splanchnic  nerve  is  formed  by  the  union  of  branches  from  the 
sixth,  seventh,  eighth,  and  ninth  ganglia;  it  passes  through  the  diaphragm  to 
the  semilunar  ganglion. 

The  lesser  splanchnic  nerve  is  formed  by  the  union  of  filaments  from  the 
tenth  and  eleventh  ganglia,  and  is  distributed  to  the  celiac  plexus. 

The  renal  splanchnic  nerve  arises  from  the  last  thoracic  ganglion  and 
terminates  in  the  renal  plexus. 

The  semilunar  ganglia,  the  largest  of  the  sympathetic  system,  are  situated 
by  the  side  of  the  celiac  axis;  they  send  radiating  branches  to  form  the  solar 


CRANIAL    NERVES.  1 95 

plexus;  from  the  various  plexuses,  nerves  follow  the  gastric,  splenic,  hepatic, 
renal,  etc.,  arteries,  into  the  different  abdominal  viscera. 

The  lumbar  ganglia,  four  in  number,  are  placed  upon  the  bodies  of  the 
vertebra ; "they  give  off  branches,  which  unite  to  form  the  aortic  lumbar  plexus 
and  the  hypogastric  plexus,  and  follow  the  blood-vessels  to  their  terminations. 

The  sacral  and  coccygeal  ganglia  send  filaments  of  distribution  to  all 
the  blood-vessels  of  the  pelvic  viscera. 

Properties  and  Functions. — The  sympathetic  nerve  possesses  both 
sensibility  and  the  power  of  exciting  motion,  but  these  properties  are  much 
less  decided  than  in  the  cerebro-spinal  system.  Division  of  the  sympathetic 
nerve  in  the  neck  is  followed  by  a  vascular  congestion  of  the  parts  above  the 
section  on  the  corresponding  side,  attended  by  an  increase  in  the  temperature; 
not  only  is  there  an  increase  in  the  amount  of  blood,  but  the  rapidity  of  the 
blood  current  is  very  much  accelerated  and  the  blood  in  the  veins  becomes  of 
a  brighter  color.  Galvanization  of  the  upper  end  of  the  divided  nerve  causes 
all  the  preceding  phenomena  to  disappear;  the  congestion  decreases,  the 
temperature  falls,  and  the  venous  blood  becomes  dark  again. 

The  sympathetic  exerts  a  similar  influence  upon  the  circulation  of  the 
limbs  and  the  glandular  organs;  destruction  of  the  first  thoracic  ganglion  and 
division  of  the  nerves  forming  the  lumbar  and  sacral  plexuses  are  followed  by 
a  dilatation  of  the  vessels,  an  increased  rapidity  of  the  circulation,  and  an 
elevation  of  temperature  in  the  anterior  and  posterior  limbs;  galvanization 
of  the  peripheral  ends  of  these  nerves  causes  all  of  these  phenomena  to  dis- 
appear. Division  of  the  splanchnic  nerve  causes  a  dilatation  of  the  blood- 
vessels of  the  intestine. 

These  phenomena  of  the  sympathetic  nervous  system  are  dependent  upon 
the  presence  of  vaso-motor  nerves,  wdiich,  under  normal  circumstances,  exert 
a  tonic  influence  upon  the  blood-vessels.  These  nerves,  derived  from  the 
cerebro-spinal  system  leave  the  spinal  cord  by  the  rami  communicantes, 
enter  the  sympathetic  ganglia  around  the  cells  of  which  they  aborize.  From 
these  cells  new  fibers  arise,  which  finally  terminate  in  the  muscle  walls  of 
the  blood-vessels. 

THE  CRANIAL  NERVES. 

The  cranial  nerves  come  off  from  the  base  of  the  brain,  pass  through  fora- 
mina in  the  walls  of  the  cranium,  and  are  distributed  to  the  structures  of  the 
head,  the  face  and  in  part  to  the  organs  of  the  thorax  and  abdomen. 

According  to  the  classification  of  Soemmering,  there  are  twelve  pairs  of 
nerves,  enumerating  them  from  before  backward,  as  follows — viz.: 


196  HUMAN  PHYSIOLOGY. 

First  nerve,  or  olfactory.  Seventh  nerve,  or  facial,  portio  dura. 

Second  nerve,  or  optic.  Eighth  nerve,  or  acoustic. 

Third  nerve,  or  motor  oculi  communis.  Ninth  nerve,  or  glosso-pharyngeal. 

Fourth  nerve,  or  trochlearis.  Tenth  nerve,  or  pneumogastric. 

Fifth  nerve,  or  trigeminal.  Eleventh  nerve,  or  spinal  accessory. 

Sixth  nerve,  or  abducent.  Twelfth  nerve,  or  hypoglossal. 

The  cranial  nerves  may  also  be  classified  physiologically,  according  to 
their  function,  into  three  groups: 

1.  Nerves  of  special  sense — e.  g.,  olfactory,  optic,  acoustic,  gustatory  (glosso- 
pharyngeal and  chorda  tympani). 

2.  Nerves  of  motion — e.  g.,  motor  oculi,  pathetic,  small  root  of  the  trigeminal, 
abducent,  facial,  spinal  accessory  and  hypoglossal. 

3.  Nerves  of  general  sensibility — e.  g.,  large  root  of  the  trigeminal,  the  glosso- 
pharyngeal and  the  pneumogastric. 

ORIGINS  OF  THE  CRANIAL  NERVES. 

The  nerves  of  special  sense  have  their  origin  in  neuro-epithelial  cells  in  the 
sense  organs  with  which  they  are  connected. 

The  nerves  of  motion  have  their  origin  in  nerve  cells  situated  in  the  gray 
matter  beneath  the  floor  of  the  aqueduct  of  Sylvius  and  the  floor  of  the  fourth 
ventricle. 

The  nerves  of  general  sensibility  have  their  origin  in  the  ganglia  situated  on 
their  trunks. 

First  Nerve.    Olfactory. 

The  olfactory  nerve  is  situated  in  the  upper  third  of  the  nasal  fossa.  It 
consists  of  from  20  to  30  branches. 

Origin. — From  neuro-epithelial  cells  situated  among  the  epithelial  cells 
covering  the  mucous  membrane.  From  these  cells  the  nerve-fibers  pass 
upward  through  foramina  in  the  cribriform  plate  of  the  ethmoid  bone  and 
arborize  around  nerve-cells,  in  the  olfactory  bulb. 

The  Olfactory  Tract. — The  olfactory  tract  consists  of  both  gray  and  white 
fibers  which  pass  from  their  origin  in  the  bulb,  to  the  base  of  the  cerebrum 
where  it  divides  into  three  branches,  viz.,  an  external  white  root,  which  passes 
across  the  fissure  of  Sylvius  to  the  middle  lobe  of  the  cerebrum;  an  internal 
white  root,  which  passes  also  into  the  middle  lobe;  a  gray  root,  which  is  in 
relation  with  the  anterior  lobe.     The  white  fibers  at  least  terminate  around 


CRANIAL   NERVES.  1 97 

nerve-cells  in  the  gray  matter  of  the  pre-callosal  part  of  the  gyrus  fornicatus, 
the  gyrus  hippocampus  and  the  gyrus  uncinatus. 

Properties. — The  olfactory  nerves  do  not  give  rise  to  either  motor  or 
sensor  phenomena  when  stimulated.  When  stimulated  at  their  periphery  by 
odorous  particles,  nerve  impulses  are  developed  which,  when  conducted  to  the 
brain,  evoke  the  sensation  of  smell.  Destruction  of  the  olfactory  nerves,  the 
bulb  or  tract,  is  followed  by  a  loss  of  the  sense  of  smell. 

Function. — Presides  over  the  sense  of  smell.  Conducts  impulses  to  the 
cerebrum  which  give  rise  to  sensations  of  odor. 

Second  Nerve.    Optic. 

Origin. — The  optic  nerve  arises  from  large  nerve-cells  in  the  anterior  part 
of  the  retina.  From  this  origin  the  nerve-fibers  turn  backward  and  converge 
to  form  a  well-defined  bundle  (the  optic  nerve)  which  passes  out  of  the  eyeball, 
through  the  orbit  cavity  as  far  as  the  sella  turcica.  At  this  point  there  is  a 
union  and  partial  decussation,  in  man  at  least,  of  the  fibers,  forming  what  is 
known  as  the  optic  chiasm.  From  the  posterior  portion  of  the  chiasm  there 
passes  backward  on  either  side  a  bundle  of  nerve-fibers,  the  optic  tract.  Each 
tract  contains  nerve-fibers  which  come  from  the  temporal  two  thirds  of  the 
retina  of  the  same  side  and  the  nasal  third  of  the  retina  of  the  opposite  side. 
The  fibers  of  the  optic  tract  arborize  around  nerve-cells  in  the  external  genic- 
ulate body,  the  pulvinar,  and  the  anterior  quadrigeminal  body.  By  means 
of  the  optic  radiation,  the  nerve-cells  in  these  different  ganglia  are  brought 
into  relation  with  the  visual  center,  the  cuneus. 

Properties. — The  optic  nerves  are  insensible  to  ordinary  impressions,  and 
convey  only  the  special  impressions  of  light.  Division  of  one  of  the  nerves  is 
attended  by  complete  blindness  in  the  eye  of  the  corresponding  side. 

Hemiopia  and  Hemianopsia. — Owing  to  the  decussation  of  the  fibers  in 
the  optic  chasm,  division  of  the  optic  tract  produces  loss  of  sight  in  the  outer 
half  oi  the  eye  of  the  same  side,  and  in  the  inner  half  oi  the  eye  of  the  opposite 
side,  the  blind  part  being  separated  from  the  normal  part  by  a  vertical  line. 
The  term  hemiopia  is  applied  to  the  loss  of  function  or  paralysis  of  the  one 
half  of  the  retina;  as  a  result  of  this,  there  will  be  an  obliteration  of  the  field  of 
vision  on  the  opposite  side  to  which  the  term  hemianopsia  is  given.  If,  for 
example,  the  right  optic  tract  be  divided,  there  will  be  hemiopia  in  the  outer  half 
of  the  right  eye  and  inner  half  of  the  left  eye,  thus  causing  left  lateral  hemian- 
opsia, and  as  the  two  halves  are  affected  which  correspond  in  normal  vision, 
the  condition  is  known  as  homonymous  hemianopsia.  Lesion  of  the  anterior 
part  of  the  optic  chiasm  causes  blindness  in  the  inner  half  of  the  two  eyes. 


198  HUMAN  PHYSIOLOGY. 

Functions. — Governs  the  sense  of  sight.  Receives  and  conveys  to  the. 
brain  the  nerve  impulses  made  by  ether  vibrations  and  which  give  rise  to  the 
sensation  of  light. 

The  reflex  movements  of  the  iris  are  called  forth  by  stimulation  of  the  optic 
nerve.  When  light  falls  upon  the  retina,  the  nerve  impulse  developed  is 
carried  back  to  the  tubercula  quadrigemina,  where  it  is  transformed  into  a 
motor  impulse, which  then  passes  outward  through  the  motor  oculi  nerve  to 
the  contractile  fibers  of  the  iris  and  diminishes  the  size  of  the  pupil.  The 
absence  of  light  is  followed  by  a  dilatation  of  the  pupil. 


Third  Nerve.    The  Oculo-Motor. 

Origin. — From  several  groups  of  nerve-cells  situated  in  the  gray  matter 
beneath  the  aqueduct  of  Sylvius. 

Distribution. — From  this  origin  the  nerve-fibers  pass  forward  and  emerge 
from  the  cerebrum  at  the  inner  side  of  the  crus  cerebri.  The  nerve  then 
passes  forward,  and  enters  the  orbit  through  the  sphenoid  fissure,  where  it 
divides  into  a  superior  branch  distributed  to  the  superior  rectus  and  levator 
palpebrce  muscles;  an  inferior  branch,  sending  branches  to  the  internal  and 
inferior  recti  and  the  inferior  oblique  muscles;  filaments  also  pass  into  the 
cilary  or  ophthalmic  ganglion;  from  this  ganglion  the  cilary  nerves  arise,  which 
enter  the  eyeball  and  are  distributed  to  the  circular  fibers  of  the  iris  and  the 
ciliary  muscle.  The  third  nerve  also  receives  filaments  from  the  cavernous 
plexus  of  the  sympathetic  and  from  the  fifth  nerve. 

Properties. — Irritation  of  the  root  of  the  nerve  produces  contraction  of  the 
pupil,  internal  strabismus,  and  muscular  movements  of  the  eye,  but  no  pain. 
Division  of  the  nerve  is  followed  by  ptosis  (falling  of  the  upper  eyelid) ;  ex 
ternal  strabismus,  due  to  the  unopposed  action  of  the  external  rectus  muscle; 
paralysis  of  the  accommodation  of  the  eye;  dilatation  of  the  pupil  from  paraly- 
sis of  the  circular  fibers  of  the  iris  and  ciliary  muscle;  and  inability  to  rotate 
the  eye,  slight  protrusion,  and  double  vision.  The  images  are  crossed ;  that  of 
the  paralyzed  eye  is  a  little  above  that  of  the  second,  and  its  upper  end  in- 
clined toward  it. 

Function. — Governs  movements  of  the  eyeball  by  innervating  all  the 
muscles  except  the  external  rectus  and  superior  oblique,  influences  the  move- 
ments of  the  iris,  elevates  the  upper  lid,  influences  the  accommodation  of  the 
eye  for  distances.  Can  be  called  into  action  by  (1)  voluntary  stimuli,  (2)  by 
reflex  action  through  irritation  of  the  optic  nerve. 


CRANIAL   NERVES.  1 99 

Fourth  Nerve.     Trochlearis. 

Origin. — From  nerve-cells  situated  in  the  gray  matter  beneath  the  aque- 
duct of  Sylvius,  just  posterior  to  the  last  nucleus  of  the  third  nerve. 

Distribution. — The  nerve  enters  the  orbital  cavity  through  the  sphenoid 
fissure,  and  is  distributed  to  the  superior  oblique  muscle;  in  its  course  it 
receives  filaments  from  the  ophthalmic  branch  of  the  fifth  pair  and  the  sym- 
pathetic. 

Properties. — When  the  nerve  is  irritated,  muscular  movements  are  pro- 
duced in  the  superior  oblique  muscle,  and  the  pupil  of  the  eye  is  turned  do-am- 
ward  and  outward.  Division  or  paralysis  lessens  the  movements  and  rotation 
of  the  globe  downward  and  outward.  The  diplopia  consequent  upon  this  par- 
alysis is  homonymous,  one  image  appearing  above  the  other.  The  image  of 
the  paralyzed  eye  is  below,  its  upper  end  inclined  toward  that  of  the  sound  eye. 

Function. — Governs  the  movements  of  the  eyeball  produced  by  the  action 
of  the  superior  oblique  muscles. 

Sixth  Nerve.*    Abducent. 

Origin. — From  nerve-cells  situated  beneath  the  upper  half  of  the  floor 
of  the  fourth  ventricle. 

Distribution. — From  this  origin  the  nerve  passes  into  the  orbit  through 
the  sphenoid  fissure,  and  is  distributed  to  the  externa  rectus  muscle.  Receives 
filaments  from  the  cervical  portion  of  the  sympathetic,  through  the  carotid 
plexus,  and  spheno-palatine  ganglion. 

Properties. — When  irritated,  the  external  rectus  muscle  is  thrown  into  con- 
vulsive movements  and  the  eyeball  is  turned  outward.  When  divided  or 
paralyzed,  this  muscle  is  paralyzed,  motion  of  the  eye  ball  outward  past  the 
median  fine  is  impossible,  and  the  homonymous  diplopia  increases  as  the 
object  is  moved  outward  past  this  fine.  The  images  are  upon  the  same 
plane  and  parallel.  Internal  strabismus  results  because  of  the  unopposed 
action  of  the  internal  rectus. 

Function. — To  innervate  the  external  rectus  muscle  by  which  the  eye- 
ball is  turned  outward. 

Fifth  Nerve.    Trigeminal. 

The  fifth  nerve  consists  of  both  afferent  and  efferent  fibers  which  for 
the  most  part  are  separate  and  distinct.  The  afferent  fibers  constitute  by 
far  the  major  portion,  the  efferent  fibers  the  minor  portion  of  the  nerve. 

*The  sixth  nerve  is  considered  in  connection  with  the  third  and  fourth  nerves  since 
they  together  constitute  the  motor  apparatus  by  which  the  ocular  muscles  are  ex- 
cited to  action. 


200  HUMAN  PHYSIOLOGY. 

Origin  of  the  Afferent  Fibers. — The  afferent  fibers  have  their  origin  in 
nerve-cells  in  the  Gasserian  ganglion.  From  each  cell  a  short  process 
develops  which  soon  divides  into  two  branches,  one  of  which  passes  centrally, 
the  other  peripherally.  The  centrally  directed  branches  form  the  so-called 
large  root;  the  peripherally  directed  branches  collectively  constitute  the  three 
main  divisions  of  the  nerve,  viz.:  the  ophthalmic,  the  superior  maxillary  and 
the  inferior  maxillary. 

Distribution. — The  centrally  directed  branches  enter  the  pons  Varolii 
on  its  lateral  aspect.  After  pursuing  a  short  distance,  these  fibers  arborize 
around  nerve-cells  in  the  gray  matter  of  the  pons  and  medulla. 

The  peripherally  directed  brances  are  distributed  as  follows: 
i.  The  ophthalmic  branches  to  the  conjunctiva  and  skin  of  the  upper  eyelid, 

trie  cornea,  the  skin  of  the  forehead  and  the  nose,  the  lachrymal  gland  and 

the  mucous  membrane  of  the  nose. 

2.  The  superior  maxillary  branches  to  the  skin  and  conjunctiva  of  the  lower 
lid,  the  nose,  the  cheek  and  upper  lip,  the  palate  teeth  of  the  upper  jaw 
and  the  alveolar  processes. 

3.  The  inferior  maxillary  branches  to  the  external  auditory  meatus,  the  side 
of  the  head,  the  mouth,  the  tongue,  the  teeth  of  the  lower  jaw,  the  alveolar 
processes  and  the  skin  of  the  lower  part  of  the  face. 

Properties. — The  trigeminal  nerve,  composed  mainly  of  afferent  fibers, 
is  the  most  acutely  sensitive  nerve  in  the  body,  and  endows  all  the  parts  to 
which  it  is  distributed  with  general  sensibility. 

Stimulation  of  the  large  root,  or  of  any  of  its  branches,  will  give  rise  to 
marked  evidence  of  pain;  the  various  forms  of  neuralgia  of  the  head  and 
face  being  occasioned  by  compression,  disease,  or  exposure  of  some  of 
its  terminal  branches. 

Division  of  the  large  root  within  the  cranium  is  followed  at  once  by  a  com- 
plete abolition  of  all  sensibility  in  the  head  and  face,  but  is  not  attended  by 
any  loss  of  motion.  The  integument,  the  mucous  membranes,  and  the  eye 
may  be  lacerated,  cut,  or  bruised,  without  the  animal  exhibiting  any  evidence 
of  pain.  At  the  same  time  the  lachrymal  secretion  is  diminished,  the  pupil 
becomes  contracted,  the  eyeball  is  protruded,  and  the  sensibility  of  the  tongue 
is  abolished. 

The  reflex  movements  of  deglutition  are  also  somewhat  impaired,  the  im- 
pression of  the  food  being  unable  to  reach  and  excite  the  nerve  center  in  the 
medulla  oblongata. 

Origin  of  the  Efferent  Fibers. — The  efferent  fibers  have  their  origin  in 
nerve-cells  in  the  gray  matter  of  the  pons  Varolii  beneath  the  floor  of  the 
fourth  ventricle. 


CRANIAL   NERVES.  201 

Distribution. — The  efferent  fibers,  known  collectively  as  the  small  root, 
emerge  from  the  side  of  the  pons  Varolii,  pass  forward  beneath  the  ganglion 
of  Gasser,  beyond  which  they  enter  the  inferior  maxillary  division.  After 
a  short  course  most  of  these  fibers  leave  the  common  trunk  and  are  distributed 
to  the  muscles  of  mastication,  viz.:  the  temporal,  the  masseter,  the  internal 
and  external  pterygoid  muscles.  Other  fibers  are  distributed  to  the  mylo- 
hyoid muscle,  the  tensor  palati  and  the  tensor  tympani  muscles. 

Properties. — Stimulation  of  the  small  root  produces  convulsive  movements 
of  the  muscles  of  mastication;  section  of  the  root  causes  paralysis  of  these 
muscles,  after  which  the  jaw  is  drawn  to  the  opposite  side  by  the  action  of 
the  opposing  muscles. 

The  Influence  of  the  Trigeminal  on  the  Special  Senses. — After  division 
of  the  large  root  within  the  cranium,  a  disturbance  in  the  nutrition  of  the 
special  senses  sooner  or  later  manifests  itself. 

Sight. — In  the  course  of  twenty-four  hours  the  eye  becomes  very  vascular 
and  inflamed,  the  cornea  becomes  opaque  and  ulcerates,  the  humors  are  dis- 
charged, and  the  eye  is  totally  destroyed. 

Smell. — The  nasal  mucous  membrane  swells  up,  becomes  fungous,  and  is 
liable  to  bleed  on  the  slightest  irritation.  The  mucus  is  increased  in  amount, 
so  as  to  obstruct  the  nasal  passages;  the  sense  of  smell  is  finally  abolished. 

Hearing. — At  times  the  hearing  is  impaired  from  disorders  of  nutrition 
in  the  middle  ear  and  external  auditory  meatus. 

Alteration  in  the  nutrition  of  the  special  senses  is  not  marked  if  the  section 
is  made  posterior  to  the  ganglion  of  Gasser  and  to  the  anastomosing  filaments 
of  the  sympathetic,  which  join  the  nerves  at  this  point;  but  if  the  ganglion 
be  divided,  these  effects  are  very  noticeable,  owing  to  the  section  of  the 
sympathetic  filaments. 

Function. — The  trigeminal  nerve,  through  its  afferent  fibers,  endows  all 
the  parts  of  the  head  and  face  to  which  it  is  distributed  with  sensibility; 
through  its  efferent  fibers  it  gives  motion  to  the  muscles  of  mastication,  and 
to  the  tensor  muscle  of  the  palate  and  the  tensor  of  the  tympanic  membrane; 
through  anastomosing  fibers  from  the  sympathetic  it  influences  the  nutrition 
of  the  special  senses. 

Seventh  Nerve.    Facial  Nerve. 

Origin. — From  a  large  nucleus  of  nerve-cells  situated  in  the  gray  matter 
beneath  the  upper  half  of  the  floor  of  the  fourth  ventricle. 

Distribution. — From  this  origin  the  nerve  emerges  from  the  lower 
border  of  the  pons.      It  then  passes  into  the  internal  auditory  meatus  in 


202  HUMAN  PHYSIOLOGY. 

company  with   the  nerve  of  Wrisberg,  and  then  enters  the  aqueduct  of 
Fallopius. 

The  nerve-fibers  composing  the  nerve  of  Wrisberg  have  their  origin  in 
nerve-cells  in  the  geniculate  ganglion,  situated  on  the  facial  just  where  it 
bends  to  enter  the  aqueduct  of  Fallopius.  The  centrally  directed  branches 
enter  the  medulla  oblongata  around  the  nerve-cells,  of  which  they  terminate; 
the  peripherally  directed  branches  enter  the  trunk  of  the  facial. 

In  the  aqueduct  the  facial  gives  off  the  following  branches — viz.: 
i.  The  large  petrosal  nerve,  which  passes  forward  to  the  splenopalatine,  or 

Meckel's  ganglion. 

2.  The  small  petrosal  nerve,  which  passes  to  the  otic  ganglion. 

3.  The  tympanic  branch,  which  passes  to  the  stapedius  muscle  and  endows  it 
with  motion. 

4.  The  chorda  tympani  nerve,  which,  after  entering  the  posterior  part  of  the 
tympanic  cavity,  passes  forward  between  the  malleus  and  incus,  through 
the  Glasserian  fissure,  and  joins  the  lingual  branch  of  the  fifth' nerve.  It 
is  then  distributed  to  the  mucous  membrane  of  the  anterior  two-thirds  of 
the  tongue  and  the  submaxillary  glands. 

After  emerging  from  the  stylomastoid  foramen,  the  facial  nerve  sends 
branches  to  the  muscles  of  the  ear,  the  occipitofron talis,  the  digastric,  the 
palatoglossi,  and  palatopharyngeal;  after  which  it  passes  through  the  parotid 
gland  and  divides  into  the  temporofacial  and  cervicofacial  branches,  which  are 
distributed  to  the  superficial  muscles  of  the  face — viz.,  occipitofron  talis,  cor- 
rugator  supercilii,  orbicularis  palpebrarum,  levator  labii  superioris  et  alaeque 
nasi,  buccinator,  levator  anguli  oris,  orbicularis  oris,  zygomatici,  depressor 
anguli  oris,  platysma  myoides,  etc. 

Properties. — The  facial  is  a  motor  nerve  at  its  origin,  but  in  its  course 
receives  sensitive  filaments  from  the  fifth  pair  and  the  pneumogastric. 

Stimulation  of  the  nerve,  after  its  emergence  from  the  stylomastoid  foramen, 
produces  convulsive  movements  in  all  the  superficial  muscles  of  the  face. 
Division  of  the  nerve  at  this  point  causes  paralysis  of  these  muscles  on  the 
side  of  the  section,  constituting/acia//>ara/y.si<>,  the  phenomena  of  which  are 
a  relaxed  and  immobile  condition  of  the  same  side  of  the  face;  the  eyelids 
remain  open,  from  paralysis  of  the  orbicularis  palpebrarum ;  the  act  of  wink- 
ing is  abolished;  the  angle  of  the  mouth  droops,  and  saliva  constantly  drains 
away;  the  face  is  drawn  over  to  the  second  side;  the  face  becomes  distorted 
upon  talking  or  laughing ;  mastication  is  interfered  with,  the  food  accumulat- 
ing between  the  gums  and  cheek,  from  paralysis  of  the  buccinator  muscle; 
fluids  escape  from  the  mouth  in  drinking;  articulation  is  impaired,  the  labial 
sounds  being  imperfectly  pronounced. 


CRANIAL    NERVES.  203 

v    Properties  and  Functions  of  the  Branches  Given  off  in  the  Aqueduct 
of  Fallopius. 

1.  The  large  petrosal,  when  stimulated,  gives  rise  to  a  dilatation  of  the  blood- 
vessels and  a  secretion  from  the  mucous  membrane  of  nose,  soft  palate, 
upper  part  of  the  pharynx,  roof  of  the  mouth,  and  gums.  It  therefore 
contains  vaso-motor  and  secretor  fibers,  which  are  in  relation  with  the 
spheno-palarine  ganglion. 

2.  The  tympanic  branch  causes  the  stapedius  muscle  to  contract. 

3.  The  chorda  tympani  influences  the  circulation  of  the  blood  around,  and  the 
secretion  of  saliva  from,  the  submaxillary  glands,  and  through  the  nerve  of 
Wrisberg  endows  the  anterior  two-thirds  of  the  tongue  with  the  sense  of  taste. 
Galvanization  of  the  chorda  tympani  dilates  the  blood-vessels,  increases  the 
quantity  and  rapidity  of  the  stream  of  blood,  and  increases  the  secretion  of 
saliva.  Division  of  the  nerve  is  followed  by  contraction  of  the  vessels,  and 
arrest  of  the  secretion,  and  a  loss  of  the  sense  of  taste  on  the  same 
side.  It  therefore  contains  vaso-motor,  secretor  and  gustatory  nerve- 
fibers. 

Function. — The  facial  is  the  nerve  of  expression,  and  coordinates  the 
muscles  employed  to  delineate  the  various  emotions,  influences  the  sense  of 
taste  and  the  secretions  of  the  submaxillary  and  sublingual  glands. 

Eighth  Nerve.    Acoustic  Nerve. 

The  eighth  nerve  consists  of  two  portions,  a  cochlear  or  auditory  and  a 
vestibular  or  equilibratory. 

Origin. — The  cochlear  portion  has  its  origin  in  the  bipolar  nerve-cells  of 
the  spinal  ganglion  located  in  the  spiral  canal  near  the  base  of  the  osseous 
lamina  spiralis.  The  vestibular  portion  has  its  origin  in  the  bipolar  nerve- 
cells  of  the  ganglion  of  Scarpa  located  in  the  internal  auditory  meatus. 

Distribution. — The  common  trunk  of  the  eighth  nerve,  consisting  of  both 
the  cochlear  and  vestibular  portions,  emerge  from  the  internal  audi  to  ry 
meatus  after  which  it  passes  backward  and  inward  as  far  as  the  lateral  aspect 
of  the  pons,  where  the  two  main  divisions  again  separate.  The  cochlear' 
portion  passes  to  the  outer  side  of  the  restiform  body;  the  vestibular  portion 
passes  to  the  inner  side  of  the  restiform  body  to  the  dorsal  portion  of  the  pons. 
After  entering  the  pons  the  fibers  composing  both  portions  come  into  histo 
logic  relations  with  different  groups  of  nerve-cells. 

Properties. — Stimulation  of  the  cochlear  nerve  is  unattended  by  eithei 
motor  or  sensor  phenomena.  Division  of  the  nerve  is  followed  by  a  loss  of 
hearing.     Destruction  of  the  semicircular  canal,  involving  a  lesion  of  the 


204  HUMAN  PHYSIOLOGY. 

vestibular  nerves  at  their  origin,  is  followed  by  a  loss  of  the  power  of  coordina* 
tion  and  equilibration. 

Functions. — The  cochlear  nerve  presides  over  the  sense  of  hearing.  It 
carries  to  the  brain  the  nerve  impulses  produced  by  the  impact  of  atmospheric 
vibrations  on  the  ear,  and  which  give  rise  to  the  sensation  of  sound.  The 
vestibular  nerve  carries  nerve  impulses  to  the  brain,  which  excite  certain 
reflex  adaptive  movements  by  which  the  equilibrium  of  the  body  is  maintained. 

Ninth  Nerve.    Glossopharyngeal. 

Origin. — From  nerve-cells  in  the  ganglia  situated  on  the  trunk  of  the  nerve 
near  the  medulla  oblongata — viz.,  the  petrosal  ganglion  and  the  jugular 
ganglion.  From  these  cells  a  single  branch  emerges,  which  soon  divides  into 
two  branches,  one  of  which  passes  centrally,  the  other  peripherally.  The 
centrally  directed  branches  enter  the  medulla  oblongata,  where  they  terminate 
around  nerve-cells.  The  peripherally  directed  branches  collectively  form  the 
two  main  divisions  from  which  the  nerve  takes  its  name. 

The  glossopharyngeal  also  contains  efferent  nerve-fibers,  which  have  their 
origin  in  nerve-cells  beneath  the  floor  of  the  fourth  ventricle. 

Distribution. — The  trunk  of  the  nerve  passes  downward  and  forward, 
receiving  near  the  jugular  ganglion  fibers  from  the  facial  and  pneumogastric 
nerves.  It  divides  into  two  large  branches,  one  of  which  is  distributed  to  the 
base  of  the  tongue,  the  other  to  the  pharynx.  In  its  course  it  sends  filaments 
to  the  otic  ganglion;  a  tympanic  branch  which  gives  sensibility  to  the  mucous 
membrane  of  the  fenestra  rotunda,  fenestra  ovalis,  and  Eustachian  tube; 
lingual  branches  to  the  base  of  the  tongue;  palatal  branches  to  the  soft  palate, 
uvula,  and  tonsils;  pharyngeal  branches  to  the  mucous  membrane  of  the 
pharynx. 

Properties. — Irritation  of  the  roots  at  their  origin  calls  forth  evidences 
of  pain;  it  is,  therefore,  a  sensor  nerve,  but  its  sensibility  is  not  so  acute  as 
that  of  the  trigeminal.  Irritation  of  the  trunk  after  its  exit  from  the  cranium 
produces  contraction  of  the  muscles  of  the  palate  and  pharynx,  owing  to  the 
presence  of  motor  fibers. 

Division  of  the  nerve  abolishes  sensibility  in  the  structures  to  which  it  is 
distributed  and  impairs  the  sense  of  taste  in  the  posterior  third  of  the  tongue 
(see  Sense  of  Taste). 

Function. — Governs  the  sensibility  of  the  pharynx,  presides  partly  over 
the  sense  of  taste,  and  controls  reflex  movements  of  deglutition  and  vomiting. 


CRANIAL   NERVES.  205 

Tenth  Nerve.    Pneumogastric.    Vagus. 

Origin. — From  the  nerve-cells  situated  along  the  trunk  of  the  nerve  near 
the  medulla  oblongata — viz.:  the  jugular  and  the  plexiform  ganglia.  From 
the  nerve-cells  in  these  ganglia  a  short  process  emerges  which  soon  divides 
into  two  branches  one  of  which  passes  centrally,  the  other  peripherally. 
The  central  branches  enter  the  medulla  oblongata,  where  they  terminate 
around  nerve-cells;  the  peripheral  branches  collectively  form  the  main  portion 
of  the  trunk  of  the  nerve. 

The  pneumogastric  also  contains  efferent  fibers  which  have  their  origin  in 
nerve-cells  beneath  the  floor  of  the  medulla  oblongata.  It  also  receives 
motor  fibers  from  the  spinal  accessory,  the  facial,  the  hypoglossal  and  the 
anterior  branches  of  the  two  upper  cervical  nerves. 

Distribution. — As  the  nerve  passes  down  the  neck  it  sends  off  the  follow- 
iny  main  branches: 

1.  Pharyngeal  nerves,  which  assist  in  forming  the  pharyngeal  plexus,  which 
is  distributed  to  the  mucous  membrane  and  to  the  muscles  of  the  pharynx. 

2.  Superior  laryngeal  nerve,  which  enters  the  larynx  through  the  thyrohyoid 
membrane,  and  is  distributed  to  the  mucous  membrane  lining  the  interior 
of  the  larynx,  and  to  the  cricothyroid  muscle  and  the  inferior  constrictor  of 
the  pharynx.  The  " depress or  nerve"  found  in  the  rabbit,  is  formed  by 
the  union  of  two  branches,  one  from  the  superior  laryngeal,  the  other  from 
the  main  trunk;  it  passes  downward  to  be  distributed  to  the  heart. 

3.  Inferior  laryngeal,  which  sends  its  ultimate  branches  to  all  the  intrinsic 
muscles  of  the  larynx  except  the  cricothyroid,  and  to  the  inferior  constrictor 
of  the  pharynx. 

4.  Cardiac  branches  given  off  from  the  nerve  throughout  its  course  which 
unite  with  the  sympathetic  fibers  to  form  the  cardiac  plexus,  to  be  distrib- 
uted to  the  heart. 

5.  Pulmonary  branches,  which  form  a  plexus  of  nerves,  and  are  distributed  to 
the  bronchi  and  their  ultimate  terminations,  the  lobules  and  air  cells. 
From   the  right  pneumogastric  nerve  branches  are  distributed   to   the 

mucous  membrane  and  the  muscular  coats  of  the  stomach  and  intestines, 
and  to  the  liver,  spleen,  kidneys,  and  suprarenal  capsules. 

Properties. — At  its  origin  the  pneumogastric  nerve  is  sensory,  as  shown 
by  direct  irritation  or  galvanization,  though  its  sensibility  is  not  very  marked. 
In  its  course  it  exhibits  motor  properties,  from  anastomosis  with  motor  nerves. 

The  pharyngeal  branches  assist  in  giving  sensibility  to  the  mucous  membrane 
of  the  pharynx,  and  influence  reflex  phenomena  of  deglutition  through  motor 
fibers  which  they  contam  in,  derived  frothe  spinal  accessory. 


2o6  HUMAN  PHYSIOLOGY. 

The  superior  laryngeal  nerve  endows  the  upper  portion  of  the  larynx  with 
sensibility;  protects  it  from  the  entrance  of  foreign  bodies;  by  conducting 
impressions  to  the  medulla,  excites  the  reflex  movements  of  deglutition  and 
respiration;  through  the  motor  filaments  it  contains,  produces  contraction  of 
the  cricothyroid  muscle. 

Division  of  the  "depressor  nerve"  and  galvanization  of  the  central  end 
retard  and  even  arrest  the  pulsations  of  the  heart,  and  by  depressing  the  vaso- 
motor center,  diminish  the  pressure  of  blood  in  the  large  vessels,  by  causing 
dilatation  of  the  intestinal  vessels  through  the  splanchnic  nerves. 

The  inferior  laryngeal  contains,  for  the  most  part,  motor  fibers  from  the 
spinal  accessory.  When  irritated,  produces  movement  in  the  laryngeal  mus- 
cles. When  divided,  is  followed  by  paralysis  of  these  muscles,  except  the 
cricothyroid,  impairment  of  phonation,  and  an  embarrassment  of  the 
respiratory  movements  of  the  larynx,  and,  finally,  death  from  suffocation. 

The  cardiac  branches,  through  filaments  derived  from  the  spinal  accessory, 
or  possibly  from  the  medulla  oblongata  direct,  exert  a  direct  inhibitory  action 
upon  the  heart.  Division  of  the  pneumogastrics  in  the  neck  is  followed 
by  increased  frequency  of  the  heart's  action.  Galvanization  of  the  peripheral 
ends  diminishes  the  heart's  pulsations,  and,  if  sufficiently  powerful,  arrests 
it  in  diastole. 

The  pulmonary  branches  give  sensibility  to  the  bronchial  mucous  membrane 
and  govern  the  movements  of  respiration.  Division  of  both  pneumogastrics 
in  the  neck  diminishes  the  frequency  of  the  respiratory  movements,  which 
may  fall  as  low  as  four  to  six  a  minute;  death  usually  occurs  in  from  five  to 
eight  days.  Feeble  galvanization  of  the  central  ends  of  the  divided  nerves 
acclerates  respiration;  powerful  galvanization  retards,  and  may  even  arrest 
the  respiratory  movements. 

The  gastric  branches  give  sensibility  to  the  mucous  coat,  and  through 
motor  or  efferent  fibers  give  motion  to  the  muscular  coat  of  the  stomach. 
They  influence  the  secretion  of  gastric  juice,  and  aid  the  process  of  digestion. 

The  hepatic  branches,  probably  through  anastomosing  sympathetic  fila- 
ments, influence  the  secretion  of  bile  and  the  glycogenic  function  of  the  liver; 
division  of  the  pneumogastrics  in  the  neck  produces  congestion  of  the  liver, 
diminishes  the  density  of  the  bile,  and  arrests  the  glycogenic  function;  gal- 
vanization of  the  central  ends  exaggerates  the  glycogenic  function  and  makes 
the  animal  diabetic. 

The  intestinal  branches  give  sensibility  and  motion  to  the  small  intestines. 

Function. — A  great  sensor  nerve,  which,  through  filaments  from  motor 
sources,  influences  deglutition,  the  action  of  the  heart,  the  circulatory  and 
respiratory  systems,  voice,  the  secretions  of  the  stomach,  intestines,  and  vari- 


CRANIAL    NERVES.  207 

ous  glandular  organs,  and  the  contraction  of  the  walls  of  the  stomach  and 
intestines. 

Eleventh  Nerve.    Spinal  Accessory. 

The  spinal  accessory  nerve  consists  of  two  distinct  portions,  the  medullary 
or  bulbar,  and  the  spinal. 

Origin. — The  medullary  portion  has  its  origin  in  nerve-cells  in  the  lower 
part  of  the  nucleus  ambiguus,  located  beneath  the  floor  of  the  fourth  ven- 
tricle. From  this  origin  the  nerve-fibers  pass  forward  and  emerge  from  the 
medulla  oblongata  on  its  lateral  aspect. 

The  spinal  portion  has  its  origin  in  the  nerve-cells  located  in  the  lateral 
gray  matter  of  the  spinal  cord  as  far  down  as  the  fifth  cervical  nerve.  From 
this  origin  the  nerve-fibers  pass  to  the  surface  of  the  cord  to  emerge  between 
the  ventral  and  dorsal  roots  in  from  six  to  eight  filaments,  after  which  they 
unite  to  form  a  well-defined  nerve. '  It  then  passes  into  the  cranial  cavity 
through  the  foramen  magnum  and  unites  with  the  medullary  portion. 

Distribution. — After  the  union  the  common  trunk  emerges  from  the  cra- 
nial cavity  through  the  jugular  foramen  and  after  sending  branches  to  the 
pneumogastric  and  receiving  others  in  turn  from  the  pneumogastric  as  well 
as  from  the  upper  cervical  nerves  it  divides  into  two  branches — viz. : 

1 .  An  internal  or  anastomotic  branch  which  soon  enters  the  trunk  of  the 
pneumogastric  nerve.  The  fibers  of  this  branch  are  ultimately  distributed 
to  some  of  the  pharyngeal  muscles;  to  all  of  the  muscles  of  the  larynx  by 
way  of  the  laryngeal  branches  of  the  vagus  nerve,  and,  according  to  most 
authorities,  to  the  heart. 

2.  An  external  branch  consisting  chiefly  of  the  accessory  fibers  from  the  spinal 
cord.     It  is  distributed  to  the  sterno-cleido-mastoid  and  trapezius  muscles. 

Properties. — At  its  origin  it  is  a  purely  motor  nerve,  but  in  its  course  it  ex- 
hibits some  sensibility,  due  to  the  presence  of  anastomosing  fibers. 

Destruction  of  the  medullary  root — e.  g.,  tearing  it  from  its  attachment  by 
means  of  forceps,  impairs  the  action  of  the  muscles  of  deglutition  and  destroys 
the  power  of  producing  vocal  sounds  from  paralysis  of  the  laryngeal  muscles, 
without,  however,  interfering  with  the  respiratory  movements  of  the  larynx, 
these  being  controlled  by  other  motor  nerves.  The  normal  rate  of  movement 
of  the  heart  is  increased  by  destruction  of  the  medullary  root. 

Irritation  of  the  external  branch  throws  the  trapezius  and  sternomastoid 
muscles  into  convulsive  movements,  though  section  of  the  nerve  does  not  pro- 
duce complete  paralysis,  as  they  are  also  supplied  with  motor  influence  from 
the  cervical  nerves.     The  sternomastoid  and  trapezius  muscles  perform  move- 


208  HUMAN  PHYSIOLOGY. 

ments  antagonistic  to  those  of  respiration,  fixing  the  head,  neck,  and  upper 
part  of  the  thorax,  and  delaying  the  expiratory  movement  during  the  acts  of 
pushing,  pulling,  straining,  etc.,  and  in  the  production  of  a  prolonged  vocal 
sound,  as  in  singing.  When  the  external  branch  alone  is  divided,  in  animals, 
they  experience  shortness  in  breath  during  exercise,  from  a  want  of  coordina- 
tion of  the  muscles  of  the  limbs  and  respiration;  and  while  they  can  make  a 
vocal  sound,  it  cannot  be  prolonged. 

Function. — Governs  phonation  by  its  influence  upon  the  muscles  regulating 
the  position  and  tension  of  the  vocal  bands;  influences  the  movements  of 
deglutition,  inhibits  the  action  of  the  heart,  and  controls  certain  respiratory 
movements  associated  with  sustained  or  prolonged  muscular  efforts  and 
phonation. 

Twelfth  Nerve.    Hypoglossal. 

Origin. — From  nerve-cells  situated  deep  in  the  substance  of  the  medulla 
oblongata,  on  a  level  with  the  lowest  portion  of  the  floor  of  the  fourth  ventricle. 
From  this  origin  the  fibers  pass  forward  and  emerge  from  the  medulla  in  the 
groove  between  the  anterior  pyramid  and  the  olivary  body. 

Distribution. — The  trunk  formed  by  the  union  of  the  different  filaments 
passes  out  of  the  cranial  cavity  through  the  anterior  condyloid  foramen. 
After  emerging  from  the  cranium,  it  sends  filaments  to  the  sympathetic  and 
pneumogastric;  it  anastomoses  with  the  lingual  branch  of  the  fifth  pair,  and 
receives  and  sends  filaments  to  the  upper  cervical  nerves.  The  nerve  is  fi- 
nally distributed  to  the  sternohyoid,  sternothyroid,  omohyoid,  thyrohyoid, 
styloglossi,  hyoglossi,  geniohyoid,  geniohyoglossi,  and  the  intrinsic  muscles  of 
the  tongue. 

Properties. — A  purely  motor  nerve  at  its  origin,  but  derives  sensibility 
outside  the  cranial  cavity  from  anastomosis  with  the  cervical  pneumogastric, 
and  fifth  nerves. 

Irritation  of  the  nerve  gives  rise  to  convulsive  movements  of  the  tongue  and 
slight  evidences  of  sensibility. 

Division  of  the  nerve  on  both  sides  abolishes  all  movements  of  the  tongue 
and  interfere  considerably  with  the  act  of  deglutition. 

When  the  hypoglossal  nerve  is  involved  in  hemiplegia,  the  tip  of  the  tongue 
is  directed  to  the  paralyzed  side  when  the  tongue  is  protruded,  owing  to  the 
unopposed  action  of  the  geniohyoglossus  on  the  sound  side. 

Articulation  is  considerably  impaired  in  paralysis  of  this  nerve,  great  diffi- 
culty being  experienced  in  the  pronunciation  of  the  consonantal  sounds. 

Mastication  is  performed  with  difficulty,  from  inability  to  retain  the  food 
between  the  teeth  until  it  is  completely  triturated. 


SENSE    OF   TOUCH.  209 

Function. — Governs  all  the  movements  of  the  tongue  and  influences  the 
functions  of  mastication,  deglutition  and  articulation. 

THE  SENSE  OF  TOUCH. 

The  sense  of  touch  is  a  modification  of  general  sensibility,  and  is  located 
in  the  skin,  which  is  especially  adapted  for  this  purpose  on  account  of  the 
number  of  nerves  and  papillary  elevations  it  possesses.  The  structures  of  the 
skin  and  the  modes  of  termination  of  the  sensory  nerves  have  already  been 
considered. 

The  tactile  sensibility  varies  in  acuteness  in  different  portions  of  the  body, 
being  most  marked  in  those  regions  in  which  the  tactile  corpuscles  are  most 
abundant — e.  g.,  the  palmar  surface  of  the  third  phalanges  of  the  fingers  and 
thumb. 

The  relative  sensibility  of  different  portions  of  the  body  has  been  ascertained 
by  means  of  a  pair  of  compasses:  the  points  of  the  instrument  being  guarded 
by  cork,  it  was  then  determined  how  closely  they  could  be  brought  together, 
and  yet  be  felt  at  two  different  points.  The  following  are  some  of  the  measure- 
ments: 

Point  of  tongue       £  of  a  line. 

Palmar  surface  of  third  phalanx 1  line. 

Red  surface  of  lips 2  lines. 

Palmar  surface  of  metacarpus 3  lines. 

Tip  of  the  nose 3  lines. 

Part  of  lips  covered  by  skin 4  lines. 

Palm  of  hand 5  lines. 

Lower  part  of  forehead 10  lines. 

Back  of  hand      14  lines. 

Dorsum  of  foot 18  lines. 

Middle  of  the  thigh 30  lines. 

The  sense  of  touch  communicates  to  the  mind  the  idea  of  resistance  only, 
and  the  varying  degrees  of  resistance  offered  to  the  sensory  nerves  enable  ur 
to  estimate,  with  the  aid  of  the  muscular  sense,  the  qualities  of  hardness  so 
softness  of  external  objects.  The  idea  of  space  or  extension  is  obtained  when 
the  sensory  surface  or  the  external  object  changes  its  place  in  regard  to  the 
other;  the  character  of  the  surface,  its  roughness  or  smoothness,  is  estimated 
by  the  impressions  made  upon  the  tactile  papillae. 

Appreciation  of  Temperature. — The  general  surface  of  the  body  is  more  or 
less  sensitive  to  differences  of  temperature,  though  this  sensation  is  separate 

14 


2IO  HUMAN  PHYSIOLOGY. 

from  that  of  touch;  whether  there  are  nerves  especially  adapted  for  the  con- 
duction of  this  sensation  has  not  been  fully  determined.  Under  pathologic 
conditions,  however,  the  sense  of  touch  may  be  abolished,  while  the  apprecia- 
tion of  changes  in  temperature  may  remain  normal. 

The  cutaneous  surface  varies  in  its  sensibility  to  temperature  in  different 
parts  of  the  body,  and  depends,  to  some  extent,  upon  the  thickness  of  the  skin, 
exposure,  habit,  etc. ;  the  inner  surface  of  the  elbow  is  more  sensitive  to  changes 
in  temperature  than  the  outer  portion  of  the  arm;  the  left  hand  is  more 
sensitive  than  the  right,  the  mucous  membrane  less  so  than  the  skin. 

Excessive  heat  or  cold  has  the  same  effect  upon  the  sensibility;  the  temper- 
atures most  readily  appreciated  are  those  between  500  F.  and  1150  F. 

The  sensations  of  pain  and  tickling  appear  to  be  conducted  to  the  brain, 
also,  by  nerves  different  from  those  of  touch;  in  abnormal  conditions  the 
appreciation  of  pain  may  be  entirely  lost  while  touch  remains  unimpaired. 

THE  SENSE  OF  TASTE. 

The  sense  of  taste  is  localized  mainly  in  the  mucous  membrane  covering 
the  superior  surface  of  the  tongue. 

The  tongue  is  situated  in  the  floor  of  the  mouth;  its  base  is  directed  back- 
ward and  is  connected  with  the  hyoid  bone  and  by  numerous  muscles  with 
the  epiglottis  and  soft  palate;  its  apex  is  directed  forward  against  the  posterior 
surface  of  the  teeth. 

The  substance  of  the  tongue  is  made  up  of  intrinsic  muscle-fibers,  the  lin- 
guales;  it  is  attached  to  surrounding  parts,  and  its  various  movements 
are  performed  by  the  extrinsic  muscles — e.  g.,  styloglossus,  geniohyo- 
glossus,  etc. 

The  mucous  membrane  covering  the  tongue  is  continuous  with  that  lining 
the  commencement  of  the  alimentary  canal,  and  is  furnished  with  vascular  and 
nervous  papillae. 

The  papillce  are  analogous  in  their  structure  to  those  of  the  skin,  and 
are  distributed  over  the  dorsum  of  the  tongue,  giving  it  its  characteristic 
roughness. 

There  are  three  principal  varieties — 

1.  The  filiform  papillce  are  most  numerous,  and  cover  the  anterior  two  thirds 
of  the  tongue;  they  are  conic  or  filiform  in  shape,  often  prolonged  into 
filamentous  tufts,  of  a  whitish  color,  and  covered  by  horny  epithelium. 

2.  The  fungiform  papillce  are  found  chiefly  at  the  tip  and  sides  of  the  tongue; 
they  are  larger  than  the  preceding,  and  may  be  recognized  by  their  deep 
red  color. 


SENSE    OF    TASTE.  211 

3.  The  circumvallate  papilla  are  rounded  eminences  from  eight  to  ten  in 
number,  situated  at  the  base  of  the  tongue,  where  they  form  a  V-shaped 
figure.  They  are  quite  large,  and  consist  of  a  central  projection  of  mucous 
membrane,  surrounded  by  a  wall,  or  circumvallation,  from  which  they 
derive  their  name. 

The  taste-beakers,  supposed  to  be  the  true  organs  of  taste,  are  flask-like 
bodies,  ovoid  in  form,  about  jfa  of  an  inch  in  length,  situated  in  the  epithelial 
covering  of  the  mucous  membrane,  on  the  circumvallate  papillae.  They  con- 
sist of  a  number  of  fusiform,  narrow  cells,  which  are  curved  so  as  to  form 
the  walls  of  this  flask-like  body;  in  the  interior  are  elongated  cells,  with  large, 
clear  nuclei,  the  taste-cells. 

Nerves  of  Taste. — The  chorda  tympani  nerve,  a  branch  of  the  facial, 
after  leaving  the  cavity  of  the  tympanum,  joins  the  third  division  of  the  fifth 
nerve  between  the  two  pterygoid  muscles,  and  then  passes  forward  in  the 
lingual  branches,  to  be  distributed  to  the  mucous  membrane  of  the  anterior 
two  thirds  of  the  tongue.  Division  or  disease  of  this  nerve  is  followed  by  a 
loss  of  taste  in  the  part  to  which  it  is  distributed. 

The  glossopharyngeal  enters  the  tongue  at  the  posterior  border  of  the 
hyoglossus  muscle,  and  is  distributed  to  the  mucous  membrane  of  the  base  and 
sides  of  the  tongue,  fauces,  etc. 

The  lingual  branch  of  the  tri-geminal  nerve  endows  the  tongue  with  general 
sensibility;  the  hypoglossal  endows  it  with  motion. 

The  nerves  of  taste  in  the  superficial  layer  of  the  mucous  membrane  form 
a  fine  plexus,  from  which  branches  pass  to  the  epithelium  and  penetrate  it; 
others  enter  the  taste-beakers,  and  are  directly  connected  with  the  taste-cells. 

The  seat  of  the  sense  of  taste  has  been  shown  by  experiment  to  be  the  whole 
of  the  mucous  membrane  over  the  dorsum  of  the  tongue,  soft  palate,  fauces, 
and  upper  part  of  the  pharynx. 

The  sense  of  taste  enables  us  to  distinguish  the  savor  of  substances  intro- 
duced into  the  mouth,  which  faculty  is  different  from  tactile  sensibility.  The 
sapid  qualities  of  substances  appreciated  by  the  tongue  are  designated  as  bit- 
ter, sweet,  alkaline,  sour,  salt,  etc. 

The  essential  conditions  for  the  production  of  the  impressions  of  taste 
are: 

1.  A  state  of  solubility  of  the  food. 

2.  A  free  secretion  of  the  saliva,  and 

3.  Active  movements  on  the  part  of  the  tongue,  exciting  pressure  against 
the  roof  of  the  mouth,  gums,  etc.,  thus  aiding  the  solution  of  various  articles 
and  their  osmosis  into  the  lingual  papillae. 


212  HUMAN  PHYSIOLOGY. 

Sapid  substances,  when  in  a  state  of  solution,  pass  into  the  interior  of  the 
taste-beakers,  and  come  into  contact,  through  the  medium  of  the  taste- 
cells,  with  the  terminal  filaments  of  the  gustatory  nerves. 

THE  SENSE  OF  SMELL. 

The  sense  of  smell  is  located  in  the  mucous  membrane  lining  the  upper 
part  of  the  nasal  cavity,  in  which  the  olfactory  nerves  are  distributed. 

The  nasal  fossae  are  two  cavities,  irregular  in  shape,  separated  by  the 
vomer,  the  perpendicular  plate  of  the  ethmoid  bone,  and  the  triangular 
cartilage.  They  open  anteriorly  and  posteriorly  by  the  anterior  and  posterior 
nares,  the  latter  communicating  with  the  pharynx.  They  are  lined  by  mucous 
membrane,  of  which  the  only  portion  capable  of  receiving  odorous  impres- 
sions is  the  part  lining  the  upper  one  third  of  the  fossae. 

The  olfactory  nerves,  the  olfactory  bulb  and  tracts,  unite  the  olfactory  ep- 
ithelium with  the  cortical  areas  of  smell  in  the  cerebrum. 

In  animals  which  possess  an  acute  sense  of  smell  there  is  a  corresponding 
increase  in  the  development  of  the  olfactory  bulbs. 

The  essential  conditions  for  the  sense  of  smell  are — 
i.  A  special  nerve  center  capable  of  receiving  impressions  and  transforming 
them  into  odorous  sensations. 

2.  Emanations  from  bodies  which  are  in  a  gaseous  or  vaporous  condition. 

3.  The  odorous  emanations  must  be  drawn  freely  through  the  nasal  fossae; 
if  the  odor  be  very  faint,  a  peculiar  inspiratory  movement  is  made,  by 
which  the  air  is  forcibly  brought  into  contact  with  the  olfactory  filaments. 
The  secretions  of  the  nasal  fossae  probably  dissolve  the  odorous  particles. 
Various  substances,  as  ammonia,  horseradish,  etc.,  excite  the  sensibility 
of  the  mucous  membrane;  this  must  be  distinguished  from  the  perception 
of  true  odors. 

THE  SENSE  OF  SIGHT. 

The  Eyeball. — The  eyeball,  or  organ  of  vision,  is  situated  at  the  fore  part 
of  the  orbital  cavity  and  is  supported  by  a  cushion  of  fat;  it  is  protected  from 
injury  by  the  bony  walls  of  the  cavity,  the  lids,  and  the  lashes,  and  is  so 
situated  as  to  permit  of  an  extensive  range  of  vision.  The  eyeball  is  loosely 
held  in  position  by  a  fibrous  membrane,  the  capsule  of  Tenon,  which  is  attached 
on  the  one  hand  to  the  eyeball  itself  and  on  the  other  to  the  walls  of  the  cavity. 
Thus  suspended,  the  eyeball  is  capable  of  being  moved  in  any  direction  by 
the  contraction  of  the  muscles  attached  to  it. 


SENSE    OF    SIGHT.  213 

Structure. — The  eyeball  is  spheroid  in  shape  and  measures  about  T?<y 
of  an  inch  in  its  anteroposterior  diameter,  and  a  little  less  in  its  transverse 
diameter.  When  viewed  in  profile,  it  is  seen  to  consist  of  the  segments  of  two 
spheres,  of  which  the  posterior  is  the  larger,  occupying  five  sixths,  and  the 
anterior  the  smaller,  occupying  one  sixth,  of  the  ball. 

The  eye  is  made  up  of  several  membranes,  concentrically  arranged,  within 
which  are  inclosed  the  refracting  media  essential  to  vision.  These  membranes 
enumerated  from  without  inward,  are — 

1.  The  sclerotic  and  cornea. 

2.  The  choroid  and  iris. 

3.  The  retina. 

The  refracting  media  are  the  aqueous  humor,  the  crystalline  lens,  and  the 
vitreous  humor. 

The  Sclerotic  and  Cornea. — The  sclerotic  is  the  opaque  fibrous  mem- 
brane covering  the  posterior  five  sixths  of  the  ball.  It  is  composed  of  con- 
nective tissue  arranged  in  layers,  which  run  both  transversely  and  longitudin- 
ally; it  is  pierced  posteriorly  by  the  optic  nerve  about  y1^  of  an  inch  internal 
to  the  optic  axis.  The  sclerotic,  by  its  density,  gives  form  to  the  eye  and 
protects  the  delicate  structures  within  it,  and  serves  for  the  attachment  of  the 
muscles  by  which  the  ball  is  moved. 

The  cornea  is  a  transparent  non- vascular  membrane  covering  the  anterior 
one  sixth  of  the  eyeball.  It  is  nearly  circular  in  shape  and  is  continuous  at 
the  circumference  with  the  sclerotic,  from  which  it  cannot  be  separated. 
The  substance  of  the  cornea  is  made  up  of  thin  layers  of  delicate,  transparent 
fibrils  of  connective  tissue,  more  or  less  united;  between  these  layers  are 
found  a  number  of  inter communi eating  lymph-spaces,  lined  by  endothelium, 
which  are  in  connection  with  lymphatics.  Leukocytes  or  lymph-corpuscles 
are  often  found  in  these  spaces.  The  anterior  surface  of  the  cornea  is 
covered  by  several  layers  of  nucleated  epithelium,  which  rest  upon  a  struc- 
tureless membrane  known  as  the  anterior  elastic  lamina.  The  posterior 
surface  is  covered  by  a  similar  membrane,  the  membrane  of  Descemet, 
which  at  its  periphery  becomes  continuous  with  the  iris;  it  is  also  covered  by 
a  layer  of  epithelial  cells.  At  the  junction  of  the  cornea  and  sclerotic  is 
found  a  circular  groove,  the  canal  of  Schlemm. 

The  choroid,  the  iris,  the  ciliary  muscle,  and  the  ciliary  processes 

together  constitute  the  second  or  middle  coat  of  the  eyeball. 

The  choroid  is  a  dark  brown  membrane  which  extends  forward  nearly  to 
the  cornea,  where  it  terminates  in  a  series  of  folds,  the  ciliary  processes.  In 
its  structure  the  choroid  is  highly  vascular,  consisting  of  both  arteries  and 
veins.     Externally  it  is  connected  with  the  sclerotic  by  connective  tissue ; 


214 


HUMAN  PHYSIOLOGY. 


internally  it  is  lined  by  a  layer  of  hexagonal  pigment  cells,  which,  though 
usually  classed  as  belonging  to  the  choroid,  is  now  known  to  belong,  embryo- 
logically  and  physiologically,  to  the  retina.     From  without  inward  may  be 
distinguished  the  following  layers: 
i.  The  lamina  suprachoroid ea. 

2 .  The  elastic  layer  of  Sattler,  consisting  of  two  endothelial  layers. 

3.  The  chorio-capillaris,  choroid  proper,  or  membrane  of  Ruysch — a  thick 
elastic  network  of  arterioles  and  capillaries  lying  within  the  outer  layer  of 
veins  and  arteries  called  the  venae  vorticosae. 

4.  The  lamina  vitrea,  or  internal  limiting  membrane. 


Fig.  32. — Sclerotic  Coat  removed  to  show  Choroid  Ciliary  Muscle,  and 

Nerves. — {From  Holden's  "Anatomy.") 

a.   Sclerotic  coat.     b.   Veins  of  the  choroid,     c.  Ciliary  nerves,     d.   Veins  of  the 

choroid,     e.  Ciliary  muscle.    /.  Iris. 


The  choroid  with  its  contained  blood-vessels  bears  an  important  relation 
to  the  nutrition  of  the  eye;  it  provides  for  the  blood-supply  and  for  drainage 
from  the  body  of  the  eye,  and  presents  a  uniform  and  high  temperature  to  the 
retina. 

The  iris  is  the  circular  variously  colored  membrane  placed  in  the  anterior 
portion  of  the  eye  just  behind  the  cornea.  It  is  perforated  a  little  to  the  nasal 
side  of  the  center  by  a  circular  opening,  the  pupil.  The  outer  or  circum- 
ferential border  is  connected  with  the  cornea,  ciliary  muscle,  and  ciliary 
processes;  the  free  inner  edge  forms  the  boundary  of  the  pupil,  the  size  of 
which  is  constantly  changing.  The  framework  of  the  iris  is  composed  of 
connective-tissue  blood-vessels,  muscle-fibers  and  pigmented  connective- 
tissue  corpuscles.     The  anterior  surface  is  covered  with  a  layer  of  epithelial 


SENSE   OF    SIGHT.  215 

cells  continuous  with  those  covering  the  posterior  surface  of  the  cornea;  the 
posterior  surface  is  lined  by  a  limiting  membrane  bearing  pigment  epithelial 
cells  continuous  with  those  of  the  choroid.  The  various  colors  which  the  iris 
assumes  in  different  individuals  depend  upon  the  quantity  and  disposition  of 
the  pigment  granules. 

The  muscle-fibers  of  the  iris,  which  are  of  the  non-striated  variety,  are 
arranged  in  two  sets — the  sphincter  and  dilatator. 

The  sphincter  pupillce  is  a  circular,  flat  band  of  muscle-fibers  surrounding 
the  pupil  close  to  its  posterior  surface;  by  its  contraction  and  relaxation  the 
pupil  is  diminished  or  increased  in  size.  The  dilatator  pupillce  consists  of  a 
thin  layer  of  fibers  arranged  in  a  radiate  manner;  at  the  margin  of  the  pupil 
they  blend  with  those  of  the  sphincter  muscle,  while  at  the  outer  border  they 
arch  to  form  a  circular  muscular  layer. 

The  ciliary  muscle  is  a  gray,  circular  band,  consisting  of  unstriped  muscle- 
fibers  about  yo  of  an  inch  long  running  from  before  backward.  It  is  at- 
tached anteriorly  to  the  inner  surface  of  the  sclerotic  and  cornea,  and  poste- 
riorly to  the  choroid  coat  opposite  the  ciliary  processes.  At  the  anterior 
border  of  the  radiating  fibers  and  internally  are  found  bundles  of  circular 
muscle-fibers,  constituting  the  annular  muscle  of  Miiller.  The  ciliary  muscle 
thus  consists  of  two  sets  of  fibers,  a  radiating  and  a  circular,  both  of  which  are 
concerned  in  effecting  a  change  in  the  convexity  of  the  lens  in  the  accommo- 
dation of  the  eye  to  near  vision. 

The  retina  forms  the  internal  coat  of  the  eye.  In  the  fresh  state  it  is  a 
delicate,  transparent  membrane  of  a  pink  color,  but  after  death  soon  becomes 
opaque;  it  extends  forward  almost  to  the  ciliary  processes,  where  it  terminates 
in  an  indented  border,  the  ora  serrata.  In  the  posterior  part  of  the  retina,  at 
a  point  corresponding  to  the  axis  of  vision,  is  a  yellow  spot,  the  macula  lutea, 
which  is  somewhat  oval  in  shape  and  tinged  with  yellow  pigment.  It  presents 
in  its  center  a  depression,  the  fovea  centralis,  corresponding  to  a  decrease  in 
thickness  of  the  retina;  about  y1^  of  an  inch  to  the  inner  side  of  the  macula  is 
the  point  of  entrance  of  the  optic  nerves.  The  arteria  centralis  retina  pierces 
the  optic  nerve  near  the  sclerotic,  runs  forward  in  its  substance,  and  is  dis- 
tributed in  the  retina  as  far  forward  as  the  ciliary  processes. 

The  retina  is  remarkably  complex,  consisting  of  ten  distinct  layers,  from 
within  outward,  supported  by  connective  tissue.  These  are  as  follows — 
viz.: 

1.  Membrana  limitans  interna. 

2.  Fibers  of  optic  nerve. 

.  3.  Layers  of  ganglionic  corpuscles. 
4.  Molecular  layer. 


2l6  HUMAN  PHYSIOLOGY. 

5.  Internal  granular  layer. 

6.  Molecular  layer. 

7.  External  granular  layer. 

8.  Membrana  limitans  externa. 

9.  Jacobson's  membrane,  or  layer  of  rods  and  cones. 
10.  The  layer  of  pigment  cells. 

The  most  important  of  these,  however,  is  the  layer  of  rods  and  cones  in  the 
external  portion  of  the  retina.  The  rods  are  straight,  elongated  cylinders 
extending  through  the  entire  thickness  of  Jacobson's  membrane.  They  con- 
sist of  an  external  portion,  which  is  clear,  homogeneous,  and  highly  refract- 
ing, and  of  an  internal  portion,  which  is  slightly  granular  and  less  refractive; 
the  outer  end  of  each  rod  is  in  direct  contact  with  the  pigment  epithelium 
lining  the  choroid,  while  the  inner  end,  tapering  to  a  fine  thread,  pierces  the 
external  limiting  membrane  and  passes  into  the  external  granular  layer.  The 
cones  consist  also  of  two  portions,  the  inner  of  which  is  somewhat  thicker 
than  the  rod  and  rests  upon  the  limiting  membrane;  the  outer  portion  tapers 
to  a  fine  point,  which  is  known  as  the  cone-style.  The  cones,  as  a  rule,  are 
somewhat  shorter  than  the  rods.  The  proportion  of  rods  to  cones  varies  in 
different  parts  of  the  retina,  though  there  are  on  an  average  about  fourteen 
rods  to  one  cone.  In  the  macula  lutea,  where  vision  is  most  acute,  the  rods 
are  almost  entirely  absent,  cones  alone  being  present.  All  the  retinal  elements 
at  this  point  are  changed.  The  nerve-fiber  layer  is  absent,  the  axis-cylinders 
radiating  in  such  a  manner  as  to  leave  the  spot  free  from  their  covering.  The 
remaining  layers  are  all  thinned  and  the  stroma  is  reduced  to  a  minimum. 
The  optic  nerve,  after  passing  forward  from  the  brain,  penetrates  in  succession 
the  sclerotic,  choroid,  and  retina;  the  nerve-fibers  then  spread  out  over  the 
anterior  surface  of  the  retina  and  become  connected  with  the  large  gangli- 
onic cells,  the  third  layer  of  the  retina. 

The  number  of  optic  nerve-fibers  in  the  retina  is  estimated  to  be  about 
800,000,  and  for  each  fiber  there  are  about  seven  cones,  one  hundred  rods,  and 
seven  pigment  cells.  The  points  of  the  rods  and  cones  are  directed  toward  the 
choroid,  or  away  from  the  entering  light,  and  dip  into  the  pigment  layer. 
They,  with  the  pigment  layer,  are  the  intermediating  elements  in  the  change 
of  the  ethereal  vibrations  into  nerve  force;  out  of  these  nerve  vibrations  the 
brain  fashions  the  sensations  of  light,  form,  and  color. 

The  vitreous  humor,  which  supports  the  retina,  is  the  largest  of  the  refracting 
media;  it  is  globular  in  form  and  constitutes  about  four  fifths  of  the  ball,  it  is 
hollowed  out  anteriorly  for  the  reception  of  the  crystalline  lens.  The  outer 
surface  of  the  vitreous  is  covered  by  a  delicate,  transparent  membrane,  termed 
the  hyaloid  membrane,  which  serves  to  maintain  its  globular  form. 


SENSE    OF    SIGHT.  217 

The  aqueous  humor,  found  in  the  anterior  chamber  of  the  eye,  is  a  clear 
alkaline  fluid,  having  a  specific  gravity  of  1003- 1009.  It  is  secreted  most 
probably  by  the  blood-vessels  of  the  iris  and  ciliary  processes.  It  passes  from 
the  interior  of  the  eye,  through  the  canal  of  Schlemm  and  the  meshes  at  the 
base  of  the  iris,  into  the  anterior  circular  vein. 

The  crystalline  lens,  inclosed  within  its  capsule,  is  a  transparent  biconvex 
body,  situated  just  behind  the  iris  and  resting  in  the  depression  in  the  anterior 
part  of  the  vitreous.  The  two  convexities  are  not  quite  alike,  the  curvature 
of  the  posterior  surface  being  slightly  greater  than  that  of  the  anterior.  The 
lens  measures  about  $  of  an  inch  in  the  transverse  diameter  and  §  of  an  inch 
in  the  anteroposterior  diameter. 

The  suspensory  ligament,  by  which  the  lens  is  held  in  position,  is  a  firm, 
transparent  membrane,  united  to  the  ciliary  processes.  A  short  distance  be- 
yond its  origin  it  splits  into  two  layers,  the  anterior  of  which  is  inserted  into 
the  capsule  of  the  lens  and  blends  with  it;  the  posterior,  passing  inward  be- 
hind the  lens,  becomes  united  to  its  capsule.  The  anterior  layer  presents  a 
series  of  foldings,  zone  of  Zinn,  which  are  inserted  into  the  intervals  of  the 
folds  of  the  ciliary  processes.  The  triangular  space  between  the  two  layers 
is  the  canal  of  Petit. 

Blood-vessels  and  Nerves. — The  structures  composing  the  eyeball  are 
supplied  with  blood  by  the  long  and  short  ciliary  arteries,  branches  of  the 
opthalmic;  they  pierce  the  sclerotic  at  various  points  and  are  ultimately  dis- 
tributed to  all  tissues  within  the  ball. 

The  nerve-supply  comes  largely  from  the  opthalmic  or  ciliary  ganglion. 
This  is  a  small  body,  situated  in  the  posterior  part  of  the  orbit;  it  receives 
motor  fibers  from  a  branch  of  the  motor  oculi,  or  third  nerve;  a  sensory  branch 
from  the  opthalmic  division  of  the  fifth  nerve,  and  fibers  from  the  cavernous 
plexus  of  the  sympathetic.  From  the  anterior  border  of  the  ganglion  proceed 
the  ciliary  nerves,  which,  entering  the  eyeball,  endow  its  structures  with  mo- 
tion and  sensation. 

The  Eyeball  a  Living  Camera  Obscura. — The  eyeball  may  be  compared 
in  a  general  way  to  a  camera  obscura.  The  anatomic  arrangement  of  its 
structures  reveals  many  points  of  similarity.  The  sclerotic  and  choroid  may 
be  compared  with  the  walls  of  the  chamber;  the  combined  refractive  media, 
cornea,  aqueous  humor,  lens,  and  vitreous  humor,  to  the  lens  for  focusing  the 
rays  of  light;  the  retina,  to  the  sensitive  plate  receiving  the  image  formed  at 
the  focal  point;  the  iris,  to  the  diaphragm,  which,  by  cutting  off  the  marginal 
rays,  prevents  spheric  aberration  and  at  the  same  time  regulates  the  amount 
of  light  entering  the  eye;  the  ciliary  muscle,  to  the  adjusting  screw,  by  which 
distinct  images  are  thrown  upon  the  retina  in  spite  of  varying  distances  of  the 


2l8 


HUMAN  PHYSIOLOGY. 


object  from  which  the  light  rays  emanate.     The  structures  just  enumerated 
are  those  essential  for  normal  vision. 

The  relationship  of  the  various  structures  composing  the  eyeball  is  shown 
in  Figure  31. 

The  dioptric  or  refracting  apparatus,  by  which  the  rays  of  light  entering 
the  eye  are  so  manipulated  as  to  produce  an  image  on  the  retina,  consists  of 
the  cornea,  aqueous  humor,  crystalline  lens,  and  vitreous  humor.  A  ray 
of  light  in  passing  through  each  of  these  media  will  undergo  refraction  at 


Fig.  S3- — Diagram  of  a  Vertical  Section  of  the  Eye. — 

(From  Holden's  "Anatomy.") 

1.  Anterior   chamber   filled   with   aqueous   humor.     2.  Posterior   chamber.     3. 

Canal  of  Petit,     a.  Hyaloid  membrane,     b.  Retina  (dotted  line),     c.  Choroid  coat 

(black    line),     d.  Sclerotic    coat.     e.  Cornea.    /.  Iris.     g.  Ciliary     processes,    h. 

Canal  of  Schlemm  or  Fontana.     i.  Ciliary  muscle. 


their  surfaces  and  ultimately  be  brought  to  a  focus  at  the  retina.  Inasmuch  as 
the  two  surfaces  of  the  cornea  are  parallel  and  its  refractive  power  practically 
the  same  as  the  aqueous  humor,  the  media  may  be  reduced  to  three — viz.: 

1.  Cornea  and  aqueous  humor. 

2.  The  lens. 

3.  The  vitreous  humor. 

The  refracting  surfaces  may  also  be  reduced  to  three — viz.: 

1.  Anterior  surface  of  the  cornea. 

2.  Anterior  surface  of  lens. 

3.  Posterior  surface  of  lens. 


SENSE    OF    SIGHT. 


219 


The  refraction  effected  by  the  cornea  is  very  great,  owing  to  the  passage 
of  the  light  from  the  air  into  a  comparatively  dense  medium,  and  is  sufficient 
of  itself  to  bring  parallel  rays  of  fight  to  a  focus  about  ten  millimeters  behind 
the  retina.  This  would  be  the  condition  in  an  eye  in  which  the  lens  was 
congenitally  absent.  Perfect  vision  requires,  however,  that  the  convergence 
of  the  light  shall  be  great  enough  to  allow  the  image  to  fall  upon  the  retina. 
This  is  accomplished  by  the  crystalline  lens,  a  body  denser  than  the  cornea 


Fig.  34. — Diagram  showing  the  Course  of  Parallel  Rays  of  Light  from,  A  , 
in  their  Passage  through  a  Biconvex  Lens,  L,  in  which  they  are  so  Refracted 
as  to  Bind  Toward  and  Come  in  a  Focus  at  a  Point,  F. — {From  Yeo's  "  Text- 
book of  Physiology.") 


Fig.  35. — Diagram  showing  the  Course  of  Diverging  Rays  which  are  Bent 
to  a  Point  Further  from  the  Lens  than  the  Parallel  Rays  in  Preceding 
Figure. — (Yeo's  "Text-book  of  Physiology.") 

and  possessing  a  higher  refractive  power.  The  manner  in  which  a  bicon 
vex  lens  focuses  both  parallel  and  divergent  rays  is  shown  in  figures  32 
and  33. 

The  function  of  the  crystalline  lens,  therefore,  is  to  focus  the  rays  of  light 
with  the  formation  of  an  image  on  the  retina. 

The  retinal  image  corresponds  in  all  respects  to  the  object  from  which 
the  light  proceeds.  The  existence  of  this  image  can  be  demonstrated  by 
removing  from  the  eye  of  a  recently  killed  animal  a  circular  portion  of  the 
sclerotic  and  choroid  posteriorly,  and  then  placing  at  the  proper  distance 
in  front  of  the  cornea  a  lighted  candle;  an  inverted  image  of  the  candle  will 
be  seen  upon  the  retina.  The  size  of  the  retinal  image  depends  upon  the 
visual  angle,  which  in  turn  depends  upon  the  size  of  the  object  and  its  dis- 
tance from  the  eye.  At  a  distance  of  15.2596  meters  the  image  of  an  ob- 
ject one  meter  high  would  be  one  millimeter,  or  a  thousand  times  smaller 
than  the  object. 


220  HUMAN  PHYSIOLOGY. 

Accommodation. — By  accommodation  is  understood  the  power  which 
the  eye  possesses  of  adjusting  itself  to  vision  at  different  distances.  In  a 
normal  or  emmetropic  eye  parallel  rays  of  light  are  brought  to  a  focus  on  the 
retina;  but  divergent  rays — that  is,  rays  coming  from  a  near  luminous  point — 
will  be  brought  to  a  focus  behind  the  retina,  provided  the  refractive  media 
remain  the  same;  as  a  result,  vision  would  be  indistinct,  from  the  formation 
of  diffusion  circles.  It  is  impossible  to  see  distinctly,  therefore,  a  near  and  a 
distant  object  at  the  same  time.  We  must  alternately  direct  the  vision  from 
one  to  the  other.  A  normal  eye  does  not  require  adjusting  for  parallel  rays; 
but  for  divergent  rays  a  change  in  the  eye  is  necessitated;  this  is  termed 
accommodation.  In  the  accommodation  for  near  vision  the  lens  becomes 
more  convex,  particularly  on  its  anterior  surface.  The  increase  in  convexity 
augments  its  refractive  power;  the  greater  the  degree  of  divergence  of  the 
rays  previous  to  entering  the  eye,  the  greater  the  increase  of  convexity  of 
the  lens  and  convergence  of  the  rays  after  passing  through  it.  By  this  altera- 
tion in  the  shape  of  the  lens  we  are  enabled  to  focus  light  rays  coming  from, 
and  to  see  distinctly,  near  as  well  as  distant  objects. 

Function  of  the  Ciliary  Muscle. — Though  it  is  admitted  that  the  change 
in  the  convexity  of  the  lens  is  caused  by  the  contraction  of  the  ciliary  muscle 
and  the  relaxation  of  the  suspensory  ligament,  the  exact  manner  in  which  it 
does  so  is  not  understood.  When  the  eye  is  in  repose,  as  in  distant  vision, 
the  suspensory  ligament  is  tense,  and  the  lens  possesses  that  degree  of  curva- 
ture necessary  for  focusing  parallel  rays.  In  the  voluntary  efforts  to  accom- 
modate the  eye  for  near  vision,  the  ciliary  muscle  contracts,  the  suspensory 
ligament  relaxes,  and  the  lens,  inherently  elastic,  bulges  forward  and  once 
again  focuses  the  rays  upon  the  retina.  It  is,  therefore,  termed  the  muscle 
of  accommodation,  and  by  its  alternate  contraction  and  relaxation  the  lens 
is  rendered  more  or  less  convex,  according  to  the  requirements  for  near  and 
distant  vision. 

Range  of  Accommodation. — Parallel  rays  coming  from  a  luminous  point 
distant  not  less  than  200  feet  do  not  require  adjustment;  from  this  point  up  to 
infinity  no  accommodation  is  required  for  perfect  vision.  This  is  termed 
the  punctum  remotum,  and  indicates  the  distance  to  which  an  object  may  be 
removed  and  yet  distinctly  seen.  If  the  object  be  brought  nearer  to  the  eye 
than  200  feet,  the  accommodative  power  must  come  into  play;  the  nearer 
the  object,  the  more  energetic  must  be  the  contraction  of  the  ciliary  muscle 
and  the  consequent  increase  in  the  convexity  of  the  lens.  At  a  distance  of 
five  inches,  however,  the  power  of  accommodation  reaches  its  maximum; 
this  is  termed  the  punctum  proximum,  and  indicates  the  nearest  point  at  which 


SENSE    OF    SIGHT.  221 

an  object  may  be  seen  distinctly.    The  distance  between  these  two  points  is 
the  range  of  accommodation. 

Optic  Defects. — Astigmatism  is  a  condition  of  the  eye  which  prevents 
vertical  and  horizontal  lines  from  being  focused  at  the  same  time,  and  is 
due  to  a  greater  curvature  of  the  cornea  in  one  meridian  than  in  another. 

Spheric  aberration  is  a  condition  in  which  there  is  an  indistinctness  of  an 
image  from  the  unequal  refraction  of  the  rays  of  light  passing  through  the 
circumference  and  the  center  of  the  lens;  it  is  corrected  mainly  by  the  iris, 
which  cuts  off  the  marginal  rays,  and  transmits  only  those  passing  through 
the  center. 

Chromatic  aberration  is  a  condition  in  which  the  image  is  surrounded  by  a 
colored  margin,  from  the  decomposition  of  the  rays  of  light  into  their  elemen- 
tary parts. 

Myopia,  or  shortsightness,  is  caused  by  an  abnormal  increase  in  the  antero- 
posterior diameter  of  the  eyeball,  or  by  a  hypernormal  refracting  power  of 
the  lens.  It  is  generally  due  to  the  first  cause;  the  lens,  being  too  far  removed 
from  the  retina,  forms  the  image  in  front  of  it,  and  the  perception  becomes 
dim  and  blurred.  Concave  glasses  correct  this  defect  by  preventing  the  rays 
from  converging  too  soon. 

Hypermetropia,  or  longsightness,  is  caused  by  a  shortening  of  the  antero- 
posterior diameter  or  by  a  subnormal  refractive  power  of  the  lens;  the  focus 
of  the  rays  of  light  would,  therefore,  be  behind  the  retina.  Convex  glasses 
correct  this  defect  by  converging  the  rays  of  light  more  anteriorly. 

Presbyopia  is  a  loss  of  the  power  of  accommodation  of  the  eye  to  near 
objects,  and  usually  occurs  between  the  ages  of  forty  and  sixty;  it  is  remedied 
by  the  use  of  convex  glasses. 

The  Iris. — The  iris  plays  the  part  of  a  diaphragm,  and  by  means  of  its 
central  aperture  the  pupil  regulates  the  quantity  of  light  entering  the  interior 
of  the  eye;  by  preventing  rays  from  passing  through  the  margin  of  the  lens 
it  diminishes  spheric  aberration.  The  size  of  the  pupil  depends  upon  the 
relative  degree  of  contraction  of  the  circular  and  radiating  fibers;  the  varia- 
tions in  size  of  the  pupil  from  variations  in  the  degree  of  contraction  depend 
upon  different  intensities  of  light.  If  the  light  be  intense,  the  circular  fibers 
contract,  and  diminish  the  size  of  the  pupil;  if  the  light  diminishes  in  inten- 
sity, the  circular  fibers  relax  and  the  pupil  enlarges. 

Point  of  Most  Distinct  Vision. — While  all  portions  of  the  retina  are 
sensitive  to  light,  their  sensibility  varies  within  wide  limits.  At  the  macula 
lutea,  and  more  especially  in  its  most  central  depression,  the  fovea,  where  the 
retinal  elements  are  reduced  practically  to  the  layer  of  rods  and  cones,  the 


22  2  HUMAN  PHYSIOLOGY. 

sensibility  reaches  its  maximum.  It  is  at  this  point  that  the  image  is  found 
when  vision  is  most  distinct.  The  macula  and  fovea  are  always  in  the  line 
of  direct  vision.  From  the  macula  toward  the  periphery  of  the  retina  there 
is  a  gradual  diminution  in  sensibility,  and  a  corresponding  decline  in  the 
distinctness  of  vision.  In  those  portions  of  the  retina  lying  outside  the 
macula,  the  indistinctness  of  vision  depends  not  only  on  diminished  sensi- 
bility, but  also  upon  inaccurate  focusing  of  the  rays. 

Blind  Spot. — Although  the  optic  nerve  transmits  the  impulses  excited 
in  the  retina  by  the  ethereal  vibration,  the  nerve-fibers  themselves  are  in- 
sensitive to  light.  At  the  point  of  entrance  of  the  optic  nerve,  owing  to  the 
absence  of  the  rods  and  cones,  the  rays  of  light  make  no  impression.  This 
is  the  blind  spot.  As  this  spot  is  not  in  the  line  of  vision,  no  dark  point  is 
ordinarily  observed  in  the  field  of  vision — the  circular  space  before  a  fixed 
eye  within  which  reflections  of  objects  are  perceptible. 

The  rods  and  cones  are  the  most  sensitive  portions  of  the  retina.  A  ray  of 
light  entering  the  eye  passes  entirely  through  the  various  layers  of  the  retina, 
and  is  arrested  only  upon  reaching  the  pigmentary  epithelium  in  which  the 
rods  and  cones  are  embedded.  As  to  the  manner  in  which  the  objective 
stimuli — light  and  color,  so  called — are  transformed  into  nerve  impulses,  but 
little  is  known.  It  is  probable  that  the  ethereal  vibrations  are  transformed 
into  heat,  which  excites  the  rods  and  cones.  These,  acting  as  highly  special- 
ized end  organs  of  the  optic  nerve,  start  the  impulses  on  their  way  to  the  brain, 
where  the  seeing  process  takes  place.  As  to  the  relative  function  of  the  rods 
and  cones,  it  has  been  suggested,  from  the  study  of  the  facts  of  comparative 
anatomy,  that  the  rods  are  impressed  only  by  differences  in  the  intensity  of 
light,  while  the  cones,  in  addition,  are  impressed  by  qualitative  differences  or 
color. 

Accessory  Structures. — The  muscles  which  move  the  eyeball  are  six  in 
number — the  superior  and  inferior  recti,  the  external  and  internal  recti, 
the  superior  and  inferior  oblique  muscles.  The  four  recti  muscles,  arising 
from  the  apex  of  the  orbit  pass  forward  and  are  inserted  into  the  sides  of  the 
sclerotic  coat;  the  superior  and  inferior  muscles  rotate  the  eye  around  a  hori- 
zontal axis;  the  external  and  internal  rotate  it  around  a  vertical  axis. 

The  superior  oblique  muscle,  having  the  same  origin,  passes  forward  to  the 
inner  and  upper  angle  of  the  orbital  cavity,  where  its  tendon  passes  through 
a  cartilaginous  pulley;  it  is  then  reflected  backward  and  inserted  into  the 
sclerotic  just  behind  the  transverse  diameter,  lis,  function  is  to  rotate  the 
eyeball  in  such  a  manner  as  to  direct  the  pupil  downward  and  outward. 

The  inferior  oblique  muscle  arises  at  the  inner  angle  of  the  orbit,  and  then 


SENSE    OF   HEARING.  223 

passes  outward  and  backward,  to  be  inserted  into  the  sclerotic.  Its  function 
is  to  rotate  the  eyeball  and  to  direct  the  pupil  upward  and  outward. 

By  the  associated  action  of  all  these  muscles,  the  eyeball  is  capable  of  per- 
forming all  the  varied  and  complex  movements  necessary  for  distinct  vision. 

The  eyelids,  bordered  with  short,  stiff  hairs,  shade  the  eye  and  protect  it 
from  injury.  On  the  posterior  surface,  just  beneath  the  conjunctiva,  are  the 
Meibomian  glands,  which  secrete  an  oily  fluid;  this  covers  the  edge  of  the 
lids,  and  prevents  the  tears  from  flowing  over  the  cheek. 

The  lacrymal  glands  are  ovoid  in  shape,  and  are  situated  at  the  upper  and 
outer  part  of  the  orbital  cavity;  they  open  by  from  six  to  eight  ducts  at  the 
outer  portion  of  the  upper  lids. 

The  tears,  secreted  by  the  lacrymal  glands,  are  distributed  over  the  cornea 
by  the  lids  during  the  act  of  winking,  and  keep  it  moist  and  free  from  dust. 
The  excess  of  tears  passes  into  the  lacrymal  ducts,  which  begin  by  two  minute 
orifices,  one  on  each  lid,  at  the  inner  canthus.  They  conduct  the  tears  into 
the  nasal  duct,  and  so  into  the  nose. 

THE  SENSE  OF  HEARING. 

The  ear,  or  organ  of  hearing,  is  lodged  within  the  petrous  portion  of  the 
temporal  bone.  It  may  be,  for  convenience  of  description,  divided  into 
three  portions — viz.: 

1.  The  external  ear. 

2.  The  middle  ear. 

3.  The  internal  ear  or  labyrinth. 

The  external  ear  consists  of  the  pinna,  or  auricle,  and  the  external  auditory 
canal.  The  pinna  consists  of  a  thin  layer  of  cartilage,  presenting  a  series  of 
elevations  and  depressions;  it  is  attached  by  fibrous  tissue  to  the  outer  bony 
edge  of  the  auditory  canal;  it  is  covered  by  a  layer  of  integument  continuous 
with  that  covering  the  side  of  the  head.  The  general  shape  of  the  pinna  is 
concave,  and  presents,  a  little  below  the  center,  a  deep  depression — the  concha. 
The  external  auditory  canal  extends  from  the  concha  inward  for  a  distance  of 
about  i|  inches.  It  is  directed  somewhat  forward  and  upward,  passing 
over  a  convexity  of  bone,  and  then  dips  downward  to  its  termination;  it  is 
composed  of  both  bone  and  cartilage,  and  is  lined  by  a  reflection  of  the  skin 
covering  the  pinna.  At  the  external  portion  of  the  canal  the  skin  contains 
a  number  of  tubular  glands — the  ceruminous  glands — which  in  their  con- 
formation resemble  the  perspiratory  glands.  They  secrete  the  cerumen,  or 
ear-wax. 

The  middle  ear,  or  tympanum,  is  an  irregularly  shaped  cavity  hollowed 
out  of  the  temporal  bone  and  situated  between  the  external  ear  and  the 


224  HUMAN  PHYSIOLOGY. 

labyrinth.  It  is  narrow  from  side  to  side,  but  relatively  long  in  its  vertical 
and  anteroposterior  diameters;  it  is  separated  from  the  external  auditory 
canal  by  a  membrane — the  membrana  tympani;  from  the  internal  ear  it  is 
separated  by  an  osseo-membranous  partition,  which  forms  a  common  wall 
for  both  cavities.  The  middle  ear  communicates  posteriorly  with  the  mas- 
toid cells;  anteriorly  with  the  nasopharynx,  by  means  of  the  Eustachian  tube. 
The  interior  of  this  cavity  is  lined  by  mucous  membrane  continuous  with 
that  lining  the  pharynx. 

The  membrana  tympani  is  a  thin,  translucent,  nearly  circular  membrane, 
measuring  about  §  of  an  inch  in  diameter,  placed  at  the  inner  termination 
of  the  external  auditory  canal.  The  membrane  is  inclosed  within  a  ring  of 
bone,  which  in  the  fetal  condition  can  be  easily  removed,  but  in  the  adult 
condition  becomes  consolidated  with  the  surrounding  bone.  The  membrana 
tympani  consists  primarily  of  a  layer  of  fibrous  tissue,  arranged  both  circu- 
larly and  radially,  and  forms  the  membrana  propria;  externally  it  is  covered 
by  a  thin  layer  of  skin  continuous  with  that  lining  the  auditory  canal;  inter- 
nally it  is  covered  by  a  thin  mucous  membrane.  The  tympanic  membrane 
is  placed  obliquely  at  the  bottom  of  the  auditory  canal,  inclining  at  an  angle 
of  forty-five  degrees,  being  directed  from  behind  and  above  downward  and 
inward.  On  its  external  surface  this  membrane  presents  a  funnel-shaped 
depression,  the  sides  of  which  are  somewhat  convex. 

The  Ear  Bones. — Running  across  the  tympanic  cavity  and  forming  an 
irregular  line  of  joined  levers  is  a  chain  of  bones  which  articulate  with  one 
another  at  their  extremities.  They  are  known  as  the  malleus,  incus,  and 
stapes. 

The  form  and  position  of  these  bones  are  shown  in  figure  36. 

The  malleus  consists  of  a  head,  neck,  and  handle,  of  which  the  latter  is 
attached  to  the  inner  surface  of  the  membrana  tympani;  the  incus,  or  anvil 
bone,  presents  a  concave,  articular  surface,  which  receives  the  head  of  the 
malleus;  the  stapes,  or  stirrup  bone,  articulates  externally  with  the  long  proc- 
ess of  the  incus,  and  internally,  by  its  oval  base,  with  the  edges  of  the  fora- 
men ovale. 

The  tensor  tympani  muscle  consists  of  a  fleshy,  tapering  portion,  \  of 
an  inch  in  length,  which  terminates  in  a  slender  tendon;  it  arises  from  the 
cartilaginous  portion  of  the  Eustachian  tube  and  the  adjacent  surface  of  the 
sphenoid  bone.  From  this  origin  the  muscle  passes  nearly  horizontally  back- 
ward to  the  tympanic  cavity;  just  opposite  to  the  fenestra  ovalis  its  tendon 
bends  at  a  right  angle  over  the  processus  cochleariformis,  and  then  passes 
outward  across  the  cavity,  to  be  inserted  into  the  angle  of  the  malleus  near 
the  neck. 


SENSE    OF   HEARING. 


225 


The  stapedius  muscle  emerges  from  the  cavity  of  a  pyramid  of  bone 
projecting  from  the  posterior  wall  of  the  tympanum;  the  tendon  passes  for- 
ward, and  is  inserted  into  the  neck  of  the  stapes  bone,  posteriorly,  near  its 
point  of  articulation  with  the  incus. 

The  laxator  tympani  muscle,  so  called,  is  now  generally  regarded  as  being 
ligamentous  in  nature,  and  not  muscular. 


i^-Sil 


Fig.  36. — Tympanum  and  Auditory  Ossicles  (Left)  Magnified. 
A.G.  External  meatus.  M.  Membrana  tympani,  which  is  attached  to  the  handle 
of  the  malleus,  n,  and  near  it  the  short  process,  p.  h.  Head  of  the  malleus,  a. 
Incus;  K.  its  short  process,  with  its  ligament;  1.  long  process,  s.  Sylvian  ossicle. 
S.  Stapes.  Ax,  Ax,  is  the  axis  of  rotation  of  the  ossicles;  it  is  shown  in  perspective 
and  must  be  imagined  to  penetrate  the  plane  of  the  paper,  t.  Line  of  traction  of 
the  tensor  tympani.  The  other  arrows  indicate  the  movement  of  the  ossicles  when 
the  tensor  contracts. 


The  Eustachian  tube,  by  means  of  which  a  free  communication  is  estab- 
lished between  the  middle  ear  and  the  pharynx,  is  partly  bony  and  partly 
cartilaginous  in  structure.  It  measures  about  1$  inches  in  length;  com- 
mencing at  its  opening  into  the  nasopharynx,  it  passes  upward  and  outward 
to  the  spine  of  the  sphenoid  bone,  at  which  point  it  becomes  somewhat  con- 
tracted; the  tube  then  dilates  as  it  passes  backward  into  the  middle-ear  cavity; 
it  is  lined  by  mucous  membrane,  which  is  continued  into  the  middle  ear  and 
mastoid  cells. 

15 


2  26  HUMAN  .PHYSIOLOGY. 

The  function  of  the  ear,  as  a  whole,  is  the  reception  and  transmission  of 
aerial  vibrations  to  the  terminal  organs  concealed  within  the  internal  ear, 
and  which  are  connected  with  the  auditory  nerve-fibers.  The  excitation  of 
these  end  organs  caused  by  the  impact  of  the  vibration  arouses  in  the  auditory 
nerve  impulses  which  are  then  transmitted  to  the  brain,  where  the  hearing 
process  takes  place.  In  order  to  appreciate  the  functions  of  the  individual 
parts  of  the  ear,  a  few  of  the  characteristics  of  sound  waves  must  be  kept  in 
mind. 

Sound  Waves. — All  sounds  are  caused  by  vibrations  in  the  atmosphere 
which  have  been  communicated  to  it  by  vibrating  elastic  bodies,  such  as 
membranes,  strings,  rods,  etc.  These  vibrating  bodies  produce  in  the  air  a 
to-and-fro  movement  of  its  particles,  resulting  in  a  series  of  alternate  conden- 
sations and  rarefactions,  which  are  propagated  in  all  directions.  A  complete 
oscillation  of  a  particle  of  air  forward  and  backward  constitutes  a  sound  wave. 
Musical  sounds  are  caused  by  a  succession  of  regular  waves,  which  follow  one 
another  with  a  certain  rapidity.  Noises  are  caused  by  the  impact  of  a  series 
of  irregular  waves. 

All  sound  waves  possess  intensity,  pitch,  and  equality.  The  intensity,  or 
loudness,  of  a  sound  depends  upon  the  amplitude  of  its  vibrations  or  on  the 
extent  of  its  excursion.  The  pitch  depends  upon  the  number  of  vibrations 
which  effect  the  auditory  nerve  in  a  second  of  time;  the  pitch  of  the  note  C,  the 
first  below  the  leger  line  of  the  musical  scale,  is  caused  by  256  vibrations  a 
second;  the  pitch  of  the  same  note  an  octave  higher  is  caused  by  512  vibra- 
tions a  second.  If  the  vibrations  are  too  few  a  second,  they  fail  to  be  per- 
ceived as  a  continuous  sound;  the  minimum  number  of  vibrations  capable  of 
producing  a  sound  has  been  fixed  at  sixteen  a  second;  the  highest  pitched 
musical  note  capable  of  being  heard  has  been  shown  to  be  due  to  38,000  vibra- 
tions a  second.  In  the  ascent  of  the  musical  scales  there  is,  therefore,  a  grad- 
ual increase  in  the  number  of  vibrations  and  a  gradual  increase  in  the  pitch 
of  the  sounds.  Between  the  two  extreme  limits  lies  the  range  of  audibility, 
which  embraces  eleven  octaves,  of  which  seven  are  employed  in  the  musical 
scale. 

The  quality  of  sound  depends  upon  a  combination  of  the  fundamental 
vibration  with  certain  secondary  vibrations  of  subdivisions  of  the  vibrating 
body.  These  so-called  over-tones  vary  in  intensity  and  pitch,  and  by  modify- 
ing the  form  of  the  primary  wave  produce  that  which  is  termed  the  quality 
of  sound. 

Function  of  the  Pinna  and  External  Auditory  Canal. — In  those  animal 
possessing  movable  ears  the  pinna  plays  an  important  part  in  the  collection  of 
sound  waves.     In  man,  in  whom  the  capalnlity  of  moving  the  pinna  has 


SENSE    OF   HEARING.  227 

been  lost,  it  is  doubtful  if  it  is  at  all  necessary  for  hearing.  Nevertheless  an 
individual  with  dull  hearing  may  have  the  perception  of  sound  increased  by 
placing  the  pinna  at  an  angle  of  45  degrees  to  the  side  of  the  head.  The  ex- 
ternal auditory  canal  transmits  the  sonorous  vibrations  to  the  tympanic  mem- 
brane. Owing  to  the  obliquity  of  this  canal  it  has  been  supposed  that  the 
waves,  concentrated  at  the  concha,  undergo  a  series  of  reflections  on  their 
way  to  the  tympanic  membrane,  and,  owing  to  the  position  of  this  mem- 
brane, strike  it  almost  perpendicularly. 

Function  of  the  Tympanic  Membrane. — The  function  of  the  tympanic 
membrane  appears  to  be  the  reception  of  sound  vibrations  by  being  thrown  by 
them  into  reciprocal  vibrations  which  correspond  in  intensity  and  amplitude. 
That  this  membrane  actually  reproduces  all  vibrations  within  the  range  of 
audibility  has  been  experimentally  demonstrated.  The  membrane  not  being 
fixed,  so  far  as  its  tension  is  concerned,  does  not  possess  a  fixed  fundamental 
note,  like  a  stationary  fixed  membrane,  and  is,  therefore,  just  as  well  adapted 
for  the  reception  of  one  set  of  vibrations  as  for  another.  This  is  made  pos- 
sible by  variations  in  its  tension  in  accordance  with  the  pitch  of  the  sounds. 
In  the  absence  of  all  sound  the  membrane  is  in  a  condition  of  relaxation;  with 
the  advent  of  sound  waves  possessing  a  gradual  increase  of  pitch,  as  in  the 
ascent  of  the  music  scale,  the  tension  of  the  tympanic  membrane  is  gradually 
inceased  until  its  maximum  tension  is  reached  at  the  upper  limit  of  the  range 
of  audibility.  By  this  change  in  tension  certain  tones  become  perceptible 
and  distinct,  while  others  become  indistinct  and  inaudible. 

Function  of  the  Tensor  Tympani  Muscle. — The  function  of  this  muscle 
is,  as  its  name  indicates,  to  increase  the  tension  of  the  membrane  in  accordance 
with  the  pitch  of  the  sound  wave.  The  tension  of  this  muscle  playing  over 
the  processus  cochleariformis  and  attached  at  also  a  right  angle  to  the  handle 
of  the  malleus  will,  when  the  muscle  contracts,  pull  the  handle  inward,  in- 
crease the  convexity  of  the  membrane,  and  at  the  same  time  increase  its  ten- 
sion; with  the  relaxation  of  this  muscle,  the  handle  of  the  malleus  passes  out- 
ward and  the  tension  is  diminished.  The  contractions  of  the  tensor  muscle  are 
reflex  in  character  and  excited  by  nerve  impulses  reaching  it  through  the  small 
petrosal  nerve  and  otic  ganglion.  The  number  of  nerve  stimuli  passing  to 
the  muscle  and  determining  the  degree  of  contraction  will  depend  upon  the 
pitch  of  the  sound  wave  and  the  subsequent  excitation  of  the  auditory  nerve. 
The  tensor  tympani  muscle  may  be  regarded  as  an  accomodative  apparatus 
by  which  the  tympanic  membrane  is  so  adjusted  as  to  enable  it  to  receive 
vibrations  of  varying  degrees  of  pitch. 

Function  of  the  Ossicles. — The  function  of  the  chain  of  bones  is  to  trans- 
mit the  sound  wave  across  the  tympanic  cavity  to  the  internal  ear.     The  first 


228  HUMAN  PHYSIOLOGY. 

of  these  bones,  the  malleus,  being  attached  to  the  tympanic  membrane,  will 
take  up  the  vibrations  much  more  readily  than  if  no  membrane  intervened. 
Owing  to  the  character  of  the  articulations,  when  the  handle  of  the  malleus  is 
drawn  inward,  the  position  of  the  bones  is  so  changed  that  they  form  practi- 
cally a  solid  rod,  and  are  therefore  much  better  adapted  for  the  transmission  of 
molecular  vibrations  than  if  the  articulations  remained  loose.  As  the  stapes 
bone  is  somewhat  shorter  than  the  malleus,  its  vibrations  are  slighter  than 
those  of  the  tympanic  membrane,  and  by  this  arrangement  the  amplitude  of 
the  vibrations  is  diminished,  but  their  force  increased. 

The  function  of  the  stapedius  muscle  is,  according  to  Henle,  to  fix  the 
stapes  bone  so  as  to  prevent  too  great  a  movement  from  being  communicated 
to  it  from  the  incus  and  transmitted  to  the  perilymph.  It  may  be  looked 
upon,  therefore,  as  a  protective  muscle. 

The  function  of  the  Eustachian  tube  is  to  maintain  a  free  communica- 
tion between  the  cavity  of  the  middle  ear  and  the  nasopharynx.  The  pressure 
of  air  within  and  without  the  ear  is  thus  equalized,  and  the  vibrations  of  the 
tympanic  membrane  are  permitted  to  attain  their  maximum,  one  of  the  con- 
ditions essential  for  the  reception  of  sound  waves.  The  impairment  in  the 
acuteness  of  hearing  which  is  caused  by  an  unequal  pressure  of  the  air  in  the 
middle  ear  can  be  shown — 
i.  By  closing  the  mouth  and  nose  and  forcing  air  from  the  lungs  through  the 

Eustachian  tube  into  the  ear,  producing  an  increase  in  pressure. 
2.  By  closing  the  nose  and  mouth,  and  making  efforts  at  deglutition,  which 

withdraws  the  air  from  the  ear  and  diminishes  its  pressure. 

In  both  instances  the  free  vibrations  of  the  tympanic  membrane  are  inter- 
fered with.  The  pharyngeal  orifice  of  the  Eustachian  tube  is  opened  by  the 
action  of  certain  of  the  muscles  of  deglutition — viz.,  the  levator  palati,  the 
tensor  palati,  and  the  palato-pharyngei  muscles. 

The  internal  ear,  or  labyrinth,  is  located  in  the  petrous  portion  of  the 
temporal  bone,  and  consists  of  an  osseous  and  a  membranous  portion. 

The  osseous  labyrinth  is  divisible  into  three  parts — viz.,  the  vestibule,  the 
semicircular  canals,  and  the  cochlea. 

The  vestibule  is  a  small,  triangular  cavity,  which  communicates  with  the 
middle  ear  by  the  foramen  ovule;  in  the  natural  condition  it  is  closed  by  the 
base  of  the  stapes  bone.  The  filaments  of  the  auditory  nerve  enter  the  vesti- 
bule through  small  foramina  in  the  inner  wall,  at  the  fovea  hemispherica. 

The  semicircular  canals  are  three  in  number,  the  superior  vertical,  the  in- 
ferior vertical,  and  the  horizontal,  each  of  which  opens  into  the  cavity  of  the 
vestibule  by  two  openings,  with  the  exception  of  the  two  vertical,  which  at  one 
extremity  open  by  a  Common  orifice. 


SENSE    OF   HEARING.  229 

The  cochlea  forms  the  anterior  part  of  the  internal  ear.  It  is  a  gradually 
tapering  canal,  about  3  \  inches  in  length,  which  winds  spirally  around  a 
central  axis,  the  modiolus,  two  and  one  half  times.  The  interior  of  the  cochlea 
is  partly  divided  into  two  passages  by  a  thin  plate  of  bone,  the  lamina  osseous 
spiralis,  which  projects  from  the  central  axis  two  thirds  of  the  way  across  the 
canal.  These  passages  are  termed  the  scala  vestibuli  and  the  scala  tympani, 
from  their  communication  with  the  vestibule  and  tympanum.  The  scala 
tympani  communicates  with  the  middle  ear  through  the  foramen  rotundum, 
which,  in  the  natural  condition,  is  closed  by  the  second  membrana  tympani ; 
superiorly  they  are  united  by  an  opening,  the  helicotrema. 

The  whole  interior  of  the  labyrinth,  the  vestibule,  the  semicircular  canals, 
and  the  scala  of  the  cochlea,  contains  a  clear,  limpid  fluid,  the  perilymph 
secreted  by  the  periosteum  lining  the  osseous  walls. 

The  membranous  labyrinth  corresponds  to  the  osseous  labyrinth  with 
respect  to  form,  though  it  is  somewhat  smaller  in  size. 

The  vestibular  portion  consists  of  two  small  sacs,  the  utricle  and  the  saccula. 

The  semicircular  canals  communicate  with  the  utricle  in  the  same  manner 
as  the  bony  canals  communicate  with  the  vestibule.  The  saccule  communi- 
cates with  the  membranous  cochlea  by  the  canalis  reuniens.  In  the  interior 
of  the  utricle  and  saccule,  at  the  entrance  of  the  auditory  nerve,  are  small 
masses  of  carbonate  of  lime  crystals,  constituting  the  otoliths.  Their  function 
is  unknown. 

The  membranous  cochlea  is  a  closed  tube,  commencing  by  a  blind  extremity 
at  the  first  turn  of  the  cochlea,  and  terminating  at  its  apex  by  a  blind  extremity 
also.  It  is  situated  between  the  edge  of  the  osseous  lamina  spiralis  and  the 
outer  wall  of  the  bony  cochlea,  and  follows  it  in  its  turns  around  the 
modiolus. 

A  transverse  section  of  the  cochlea  shows  that  it  is  divided  into  two  por- 
tions by  the  osseous  lamina  and  the  basilar  membrane: 

1.  The  scala  vestibuli,  bounded  by  the  periosteum  and  membrane  of  Reissner. 

2.  The  scala  tympania,  occupying  the  inferior  portion,  and  bounded  above  by 
the  septum,  composed  of  the  osseous  lamina  and  the  membrana  basilaris. 
The  true  membranous  canal  is  situated  between  the  membrane  of  Reissner 

and  the  basilar  membrane.  It  is  triangular  in  shape,  but  is  partly  divided 
into  a  triangular  portion  and  a  quadrilateral  portion  by  the  tectorial 
membrane. 

The  organ  of  Corti  is  situated  in  the  quadrilateral  portion  of  the  canal,  and 
consists  of  pillars  of  rods  of  the  consistence  of  cartilage.  They  are  arranged 
in  two  rows — the  one  internal,  the  other  external;  these  rods  rest  upon  the 
basilar  membrane;  their  bases  are  separated  from  one  another,  but  their 


230  HUMAN  PHYSIOLOGY. 

upper  extremities  are  united,  forming  an  arcade.  In  the  internal  row  it  is 
estimated  there  are  about  3,500  and  in  the  external  row  about  5,200  of  these 
rods. 

On  the  inner  side  of  the  internal  row  is  a  single  layer  of  elongated  hair-cells; 
on  the  outer  surface  of  the  external  row  are  three  such  layers  of  hair-cells. 
Nothing  definite  is  known  as  to  their  function. 

The  endolymph  occupies  the  interior  of  the  utricle,  saccule,  and  membran- 
ous canals,  and  bathes  the  structures  in  the  interior  of  the  membranous  coch- 
lea throughout  its  entire  extent. 

The  auditory  nerve  at  the  bottom  of  the  internal  auditory  meatus  divides 
into — 

1.  A  vestibular  branch,  which  is  distributed  to  the  utricle  and  to  the  semi- 
circular canals. 

2.  A  cochlear  branch,  which  passes  into  the  central  axis  at  its  base  and  as- 
cends to  its  apex;  as  is  ascends,  fibers  are  given  off,  which  pass  between 
the  plates  of  the  osseous  lamina,  to  be  ultimately  connected  with  the  organ 
of  Corti. 

The  function  of  the  semicircular  canals  appears  to  be  to  assist  in  maintaining 
the  equilibrium  of  the  body;  destruction  of  the  vertical  canal  is  followed  by 
an  oscillation  of  the  head  upward  and  downward;  destruction  of  the  horizon- 
tal canal  is  followed  by  oscillations  from  left  to  right.  When  the  canals 
are  injured  on  both  sides,  the  animal  loses  the  power  of  maintaining 
equilibrium  upon  making  muscular  movements. 

Function  of  the  Cochlea. — It  is  regarded  as  possessing  the  power  of  appreci- 
ating the  quality  of  pitch  and  the  shades  of  different  musical  tones.  The  ele- 
ments of  the  organ  of  Corti  are  analogous,  in  some  respects,  to  a  musical 
instrument,  and  are  supposed,  by  Helmholtz,  to  be  tuned  so  as  to  vibrate  in 
unison  with  the  different  tones  conveyed  to  the  internal  ear. 

Summary. — The  waves  of  sound  are  gathered  together  by  the  pinna  and 
external  auditory  meatus,  and  conveyed  to  the  membrana  tympani.  This 
membrane,  made  tense  or  lax  by  the  action  of  the  tensor  tympani  and  laxator 
tympani  muscles,  is  enabled  to  receive  sound  waves  of  either  high  or  low  pitch. 
The  vibrations  are  conducted  across  the  middle  ear  by  a  chain  of  bones  to  the 
foramen  ovale,  and  by  the  column  of  air  of  the  tympanum  to  the  foramen 
rotundum,  which  is  closed  by  the  second  membrana  tympani,  the  pressure  of 
the  air  in  the  tympanum  being  regulated  by  the  Eustachian  tube. 

The  internal  ear  finally  receives  the  vibrations,  which  excite  vibrations 
successively  in  the  perilymph,  the  walls  of  the  membranous  labyrinth,  the 
endolymph,  and,  lastly,  the  terminal  filaments  of  the  auditory  nerve,  by  which 
they  are  conveyed  to  the  brain. 


VOICE  AND    SPEECH.  23 1 

VOICE  AND  SPEECH. 

The  larynx  is  the  organ  of  voice.  Speech  is  a  modification  of  voice,  and 
is  produced  by  the  teeth  and  the  muscles  of  the  lips  and  tongue,  coordinated 
in  their  action  by  stimuli  derived  from  the  cerebrum. 

The  structures  entering  into  the  formation  of  the  larynx  are  mainly  the 
thyroid,  cricoid,  and  arytenoid  cartilages;  they  are  so  situated  and  united  by 
means  of  ligaments  and  muscles  as  to  form  a  firm  cartilaginous  box.  The 
larynx  is  covered  externally  by  fibrous  tissue,  and  lined  internally  with  mu- 
cous membrane. 

The  vocal  bands  are  four  ligamentous  bands,running  anteroposterior!}- 
across  the  upper  portion  of  the  larynx,  and  are  divided  into  the  two  superior 
or  false  vocal  bands,  and  the  tivo  inferior  or  true  vocal  bands;  they  are  attached 
anteriorly  to  the  receding  angle  of  the  thyroid  cartilages,  and  posteriorly  to 
the  anterior  part  of  the  base  of  the  arytenoid  cartilages.  The  space  between 
the  true  vocal  bands  is  the  rima  glottidis. 

The  muscles  which  have  a  direct  action  upon  the  movements  of  the  vocal 
bands  are  nine  in  number,  and  take  their  names  from  their  points  of  origin  and 
insertion — viz.,  the  two  crico-thyroid,  two  thyro-arytenoid,  two  posterior 
crico-arytenoid,  two  lateral  crico-arytenoid,  and  one  arytenoid  muscles. 

The  crico-thyroid  muscles,  by  their  contraction,  render  the  vocal  bands 
more  tense  by  drawing  down  the  anterior  portion  of  the  thyroid  cartilage 
and  approximating  it  to  the  cricoid,  and  at  the  same  time  tilting  the  posterior 
portion  of  the  cricoid  and  arytenoid  cartilages  backward. 

The  thyro-arytenoid,  by  their  contraction,  relax  the  vocal  bands  by  drawing 
the  arytenoid  cartilage  forward  and  the  thyroid  backward. 

The  posterior  crico-arytenoid  muscles,  by  their  contraction,  rotate  the  ary- 
tenoid cartilages  outward  and  thus  separate  the  vocal  bands  and  enlarge  the 
aperture  of  the  glottis.  They  principally  aid  the  respiratory  movements 
during  inspiration. 

The  lateral  crico-arytenoid  muscles  are  antagonistic  to  the  former,  and  by 
their  contraction  rotate  the  arytenoid  cartilages  so  as  to  approximate  the 
vocal  bands  and  constrict  the  glottis. 

The  arytenoid  muscle  assists  in  the  closure  of  the  aperture  of  the  glottis. 

The  inferior  laryngeal  nerve  animates  all  the  muscles  of  the  larynx,  with 
the  exception  of  the  crico-thyroid. 

Movements  of  the  Vocal  Bands. — During  respiration  the  movements  of 
the  vocal  bands  differ  from  those  occurring  during  the  production  of  voice. 

At  each  inspiration  the  true  vocal  bands  are  widely  separated,  and  the  aper- 
ture of  the  glottis  is  enlarged  by  the  action  of  the  crico-arytenoid  muscles, 
which  rotate  outward  the  anterior  angle  of  the  base  of  the  arytenoid  cartilages ; 


232  HUMAN  PHYSIOLOGY. 

at  each  expiration  the  larynx  becomes  passive;  the  elasticity  of  the  vocal  bands 
returns  them  to  their  original  position,  and  the  air  is  forced  out  by  the  elasticity 
of  the  lungs  and  the  walls  of  the  thorax. 

Phonation. — As  soon  as  phonation  is  about  to  be  accomplished,  a  marked 
change  in  the  glottis  is  noticed  with  the  aid  of  the  laryngoscope.  The  true 
vocal  bands  suddenly  become  approximated  and  are  made  parallel,  giving  to 
the  glottis  the  appearance  of  a  narrow  slit,  the  edges  of  which  are  capable 
of  vibrating  accurately  and  rapidly;  at  the  same  time  their  tension  is  much 
increased. 

With  the  vocal  bands  thus  prepared  the  expiratory  muscles  force  the  column 
of  air  into  the  lungs  and  trachea  through  the  glottis,  throwing  the  edges  of 
the  bands  into  vibration. 

The  pitch  of  sounds  depends  upon  the  extent  to  which  the  vocal  bands  are 
made  tense  and  the  length  of  the  aperture  through  which  the  air  passes.  In 
the  production  of  sounds  of  a  high  pitch,  the  tension  of  the  vocal  bands  becomes 
very  marked  and  the  glottis  diminished  in  length.  When  sounds  having  a  low 
pitch  are  emitted  from  the  larynx,  the  vocal  bands  are  less  tense  and  their 
vibrations  are  large  and  loose. 

The  quality  of  voice  depends  upon  the  length,  size,  and  thickness  of  the 
bands,  and  upon  the  size,  form,  and  construction  of  the  trachea,  the  larynx, 
and  the  resonant  cavities  of  the  pharynx,  nose,  and  mouth. 

The  compass  of  the  voice  comprehends  from  two  to  three  octaves.  The 
range  is  different  in  the  two  sexes,  the  lowest  note  of  the  male  being  about  one 
octave  lower  than  the  lowest  note  of  the  female;  while  the  highest  note  of  the 
male  is  an  octave  less  than  the  highest  note  of  the  female. 

The  varieties  of  voice — e.  g.,  bass,  baritone,  tenor,  contralto,  mezzo- 
soprano,  and  soprano — are  due  to  the  length  of  the  vocal  bands,  being  longer 
when  the  voice  has  a  low  pitch,  and  shorter  when  it  has  a  high  pitch. 

Speech  is  the  faculty  of  expressing  ideas  by  means  of  combinations  of  sounds 
in  obedience  to  the  dictates  of  the  cerebrum. 

Articulate  sounds  may  be  divided  into  vowels  and  consonants.  The 
vowel  sounds,  a,  e,  i,  0,  u,  are  produced  in  the  larynx  by  the  vocal  cords.  The 
consonant  sounds  are  produced  in  the  air-passages  above  the  larynx  by  an  in- 
terruption of  the  current  of  air  by  the  lips,  tongue,  and  teeth ;  the  consonants 
may  be  divided  into: 

1.  Mutes,  b,  d,  k,  p,  t,  c,  g. 

2.  Dentals,  d,  j,  s,  t,  z. 

3.  Nasals,  m,  n,  ng. 

4.  Labials,  b,  p,f,  v,  m. 

5.  Gutterals,  k,  g,  c,  and  g  hard. 

6.  Liquids,  I,  m,  n,  r. 


REPRODUCTION. 

Reproduction  is  the  function  by  which  the  species  is  preserved;  it  is 
accomplished  by  the  organs  of  generation  in  the  two  sexes.  Embryology 
is  the  science  which  investigates  the  successive  stages  in  the  development 
of  the  embryo. 

GENERATIVE  ORGANS  OF  THE  FEMALE. 

The  generative  organs  of  the  female  consist  of  the  ovaries,  Fallopian 
tubes,  uterus,  and  vagina. 

The  ovaries  are  two  small,  ovoid,  flattened  bodies,  measuring  i\  inches 
in  length  and  f  of  an  inch  in  width;  they  are  situated  in  the  cavity  of  the 
pelvis,  and  are  imbedded  in  the  posterior  layer  of  the  broad  ligament;  attached 
to  the  uterus  by  a  round  ligament,  and  to  the  extremities  of  the  Fallopian  tubes 
by  the  fimbriae.  The  ovary  consists  of  an  external  membrane  of  fibrous 
tissue,  the  cortical  portion,  in  which  are  embedded  the  Graafian  vesicles,  and 
an  internal  portion,  the  stroma,  containing  blood-vessels. 

The  Graafian  vesicles  are  exceedingly  numerous,  but  are  situated  only  in 
the  cortical  portion.  It  is  estimated  that  each  ovary  contains  from  20,000 
to  40,000  follicles.  Although  the  ovary  contains  the  vesicles  from  the  period 
of  birth,  it  is  only  at  puberty  that  they  attain  their  full  development.  From 
this  time  onward  to  the  catamenial  period  there  is  a  constant  growth  and 
maturation  of  the  Graafian  vesicles.  They  consist  of  an  external  investment, 
composed  of  fibrous  tissues  and  blood-vessels,  in  the  interior  of  which  is  a 
layer  of  cells  forming  the  membrana  granulosa;  at  its  lower  portion  there  is  an 
accumulation  of  cells,  the  proligerous  disc,  in  which  the  ovum  is  contained. 
The  cavity  of  the  vesicle  contains  a  slightly  yellowish  alkaline,  albuminous 
fluid. 

The  ovum  is  a  globular  body,  measuring  about  T^  of  an  inch  in 
diameter.  It  consists  of  a  mass  of  protoplasmic  material  cytoplasm,  a 
nucleus  or  germinal  vesicle  and  a  nucleolus  or  germinal  spot.  The  peripheral 
portion  of  the  cytoplasm  is  surrounded  by  a  clear  thick  membrane  the  zona 
pellucida,  external  to  which  is  a  layer  of  radially  placed  columnar  epithelium 

233 


234  HUMAN  PHYSIOLOGY. 

forming  the  corona  radiata.  The  nucleus  consists  of  a  nuclear  mem- 
brane enclosing  material,  some  of  which  arranged  in  the  form  of  thread 
stains  readily  and  hence  known  as  chromatin  in  the  meshes  of  which  lies 
a  material  and  stains  faintly  and  hence  known  as  achromatin. 

The  Fallopian  tubes  are  about  four  inches  in  length,  and  extend  outward 
from  the  upper  angles  of  the  uterus,  between  the  folds  of  the  broad  ligaments, 
and  terminate  in  a  fringed  extremity  which  is  attached  by  one  of  the  fringes 
to  the  ovary.     They  consist  of  three  coats: 
i.  The  external,  or  peritoneal. 

2.  Middle,  or  muscular,  the  fibers  of  which  are  arranged  in  a  circular  and 
longitudinal  direction. 

3.  Internal,  or  mucous,  usually  folded  longitudinally  is  covered  with  ciliated 
epithelial  cells,  which  are  always  waving  from  the  ovary  toward  the  uterus. 

The  uterus  is  pyriform  in  shape,  and  may  be  divided  into  a  body  and 
neck;  it  measures  about  three  inches  in  length  and  two  inches  in  breadth  in 
the  unimpregnated  state.  At  the  lower  extremity  of  the  neck  is  the  os  ex- 
ternum; at  the  junction  of  the  neck  with  the  body  is  a  constriction,  the  os 
internum.  The  cavity  of  the  uterus  is  triangular  in  shape,  the  walls  of  the 
triangle  being  almost  in  contact. 

The  walls  of  the  uterus  are  made  up  of  many  layers  of  non-striated  muscle- 
fibers,  covered  externally  by  peritoneum,  and  lined  internally  by  mucous 
membrane,  containing  numerous  tubular  glands,  and  covered  by  ciliated 
epithelial  cells. 

The  vagina  is  a  membranous  canal,  from  five  to  six  inches  in  length, 
situated  between  the  rectum  and  bladder.  It  extends  obliquely  upward  from 
the  surface,  almost  to  the  brim  of  the  pelvis,  and  embraces  at  its  upper  ex- 
tremity the  neck  of  the  uterus. 

Discharge  of  the  Ovum. — As  the  Graafian  vesicle  matures  it  increases 
in  size,  from  an  augmentation  of  its  liquid  contents,  and  approaches  the 
surface  of  the  ovary,  where  it  forms  a  projection,  measuring  from  \  to  \ 
of  an  inch.  The  maturation  of  the  vesicle  occurs  periodically,  about  every 
twenty-eight  days,  and  is  attended  by  the  phenomena  of  menstruation. 
During  this  period  of  active  congestion  of  the  reproductive  organs  the 
Graafian  vesicle  ruptures,  the  ovum  and  liquid  contents  escape,  and  are 
caught  by  the  fimbriated  extremity  of  the  Fallopian  tube,  which  has  adapted 
itself  to  the  posterior  surface  of  the  ovary.  The  passage  of  the  ovum  through 
the  Fallopian  tube  into  the  uterus  occupies  from  ten  to  fourteen  days,  and  is 
accomplished  by  muscular  contraction  and  by  the  action  of  the  ciliated 
epithelium. 


REPRODUCTION.  235 

Menstruation  is  a  periodic  discharge  of  blood  from  the  mucous  membrane 
of  the  uterus,  due  to  a  fatty  degeneration  of  the  small  blood-vessels.  Under 
the  pressure  of  an  increased  amount  of  blood  in  the  reproductive  organs, 
attending  the  process  of  ovulation,  the  blood-vessels  rupture,  and  a  hemor- 
rhage takes  place  into  the  uterine  cavity;  thence  it  passes  into  the  vagina. 
Menstruation  lasts  from  five  to  six  days,  and  the  amount  of  blood  discharged 
averages  about  five  ounces. 

Corpus  Luteum. — For  some  time  previous  to  the  rupture  of  a  Graafian 
vesicle  it  increases  in  size  and  becomes  vascular;  its  walls  become  thickened 
from  the  deposition  of  a  reddish-yellow,  glutinous  substance,  a  product  of 
cell  growth  from  the  proper  coat  of  the  follicle  and  the  membrana  granulosa. 
After  the  ovum  escapes  there  is  usually  a  small  effusion  of  blood  into  the 
cavity  of  the  follicle,  which  soon  coagulates,  loses  its  coloring-matter,  and 
acquires  the  characteristics  of  fibrin,  but  it  takes  no  part  in  the  formation  of 
the  corpus  luteum.  The  walls  of  the  follicle  become  convoluted  and  vascular 
and  undergo  hypertrophy,  until  they  occupy  the  whole  of  the  follicular 
cavity.  At  its  period  of  fullest  development  the  corpus  luteum  measures 
of  an  inch  in  length  and  \  of  an  inch  in  depth.  In  a  few  weeks  the  mass 
loses  its  red  color  and  becomes  yellow,  constituting  the  corpus  luteum,  or 
yellow  body.  It  then  begins  to  retract  and  becomes  pale;  and  at  the  end  of 
two  months  nothing  remains  but  a  small  cicatrix  upon  the  surface  of  the 
ovary.  Such  are  the  changes  in  the  follicle  if  the  ovum  has  not  been 
impregnated. 

The  corpus  luteum,  after  impregnation  has  taken  place,  undergoes  a  much 
slower  development,  becomes  larger,  and  continues '  during  the  entire  period 
of  gestation.  The  difference  between  the  corpus  luteum  of  the  unimpreg- 
nated  and  pregnant  condition  is  expressed  in  the  following  table  by  Dalton: 


Corpus  Luteum  of  Menstruation.     Corpus  Luteum  of  Pregnancy. 


At  the  end  of  three 

weeks. 
One  month 


Three  quarters  of  an  inch  in  diameter;  central  clot 
reddish;  convoluted  wall  pale. 

Smaller;  convoluted  Larger;  convoluted  wall 
wall  bright  yellow;  bright  yellow;  clot  still  reddish, 
clot  still  reddish. 

Two  months 1       Reduced  to  the  con-       Seven    eigths    of  an  inch  in 

dition  of  an  insignifi-     diameter;     convoluted     wall 
cant  cicatrix.  bright    yellow;     clot    perfectly 

decolorized. 


236 

Four  months 

Six  months . . 
Nine  months 


HUMAN  PHYSIOLOGY. 


Absent      or       un- 
noticeable. 


Absent . 


Absent . 


Seven  eights  of  an  inch  in 
diameter;  clot  pale  and  fibrin- 
ous; convoluted  wall  dull  yel- 
low. 

Still  as  large  as  at  the  end  of 
second  month;  clot  fibrinous; 
convoluted  wall  paler. 

Half  an  inch  in  diameter; 
central  clot  converted  into  a 
radiating  cicatrix;  external 
wall  tolerably  thick  and  con- 
voluted, but  without  any  bright 
yellow  color. 


GENERATIVE  ORGANS  OF  THE  MALE. 

The  generative  organs  of  the  male  consist  of  the  testicles,  vasa  deferen- 
tia,  vesiculae  seminales,  and  penis. 

The  testicles,  the  essential  organs  of  reproduction  in  the  male,  are  two 
oblong  glands,  about  1  \  inches  in  length,  compressed  from  side  to  side,  and 
situated  in  the  cavity  of  the  scrotum. 

The  proper  coat  of  the  testicle,  the  tunica  albuginea,  is  a  white,  fibrous 
structure,  about  ■£%  of  an  inch  in  thickness;  after  enveloping  the  testicle, 
it  is  reflected  into  its  interior  at  the  posterior  border,  and  forms  a  vertical 
process,  the  mediastinum  testis,  from  which  septa  are  given  off,  dividing  the 
testicle  into  lobules. 

The  substance  of  the  testicle  is  made  up  of  the  seminiferous  tubules,  which 
exist  to  the  number  of  840;  they  are  exceedingly  convoluted,  and  when  un- 
ravelled are  about  thirty  inches  in  length.  As  they  pass  toward  the  apices 
of  the  lobules,  they  become  less  convoluted,  and  terminate  in  from  twenty 
to  thirty  straight  ducts,  the  vasa  recta,  which  pass  upward  through  the 
mediastinum  and  constitute  the  rete  testis.  At  the  upper  part  of  the  mediasti- 
num the  lobules  unite  to  form  from  nine  to  thirty  small  ducts,  the  vasa  effer- 
entia,  which  become  convoluted  and  form  the  globus  major  of  the  epididymis; 
the  continuation  of  the  tubes  downward  behind  the  testicle  and  a  second 
convolution  constitutes  the  body  and  globus  minor. 

The  seminal  tubule  consists  of  a  basement  membrane  lined  by  granular 
nucleated  epithelium. 

The  vas  deferens,  the  excretory  duct  of  the  testicle,  is  about  two  feet  in 
length,  and  may  be  traced  upward  from  the  epididymis  to  the  under  surface 


REPRODUCTION.  237 

of  the  base  of  the  bladder,  where  it  unites  with  the  duct  of  the  vesicula  semin- 
alis  to  form  the  ejaculatory  duct. 

The  vesiculae  seminales  are  two  lobulated,  pyriform  bodies  about  two 
inches  in  length,  situated  on  the  inner  surface  of  the  bladder. 

They  have  an  external  fibrous  coat,  a  middle  muscular  coat,  and  an  internal 
mucous  coat,  covered  by  epithelium,  which  secretes  a  mucous  fluid.  The 
vesiculae  seminales  serve  as  reservoirs,  in  which  the  seminal  fluid  is  tem- 
porarily stored  up. 

The  ejaculatory   duct,   about  f   of  an  inch  in  length,  opens  into  the 
urethra,  and  is  formed  by  the  union  of  the  vasa  deferentia  and  the  ducts  of 
the  vesiculae  seminales. 

The  prostate  gland  surrounds  the  posterior  extremity  of  the  urethra,  and 
opens  into  it  by  from  twenty  to  thirty  openings,  the  orifices  of  the  prostatic 
tubules.  The  gland  secretes  a  fluid  which  forms  part  of  the  semen  and 
assists  in  maintaining  the  vitality  of  the  spermatozoa. 

Semen  is  a  complex  fluid,  made  up  of  the  scretions  from  the  testicles,  the 
vesiculae  seminales,  the  prostatic  and  urethral  glands.  It  is  grayish-white  in 
color,  mucilaginous  in  consistence,  of  a  characteristic  odor,  and  somewhat 
heavier  than  water.  From  half  a  dram  to  a  dram  is  ejaculated  at  each 
orgasm. 

The  spermatozoa  are  peculiar  anatomic  elements,  developed  within  the 
seminal  tubules,  and  possess  the  power  of  spontaneous  movement.  The  sper- 
matozoa consist  of  a  conoid  head  and  a  long,  filamentous  tail,  which  is  in 
continuous  and  active  motion;  so  long  as  they  remain  in  the  vas  deferens 
they  are  quiescent,  but  when  free  to  move  in  the  fluid  of  the  vesiculae  semi- 
nales, they  become  very  active. 

Origin. — The  spermatozoa  appear  at  the  age  of  puberty,  and  are  then 
constantly  formed  until  an  advanced  age.  They  are  developed  from  the 
nuclei  of  large,  round  cells  contained  in  the  anterior  of  the  seminal  tubules, 
as  many  as  fifteen  to  twenty  developing  in  a  single  cell. 

When  the  spermatozoa  are  introduced  into  the  vagina,  they  pass  readily 
into  the  uterus  and  through  the  Fallopian  tubes  toward  the  ovaries,  where 
they  remain  and  retain  their  vitality  for  a  period  of  from  eight  to  ten  days. 

Fecundation  is  the  union  of  the  spermatozoa  with  the  ovum  during  its 
passage  toward  the  uterus  and  usually  takes  place  in  the  Fallopian  tube 
just  outside  the  uterus.  After  floating  around  the  ovum  in  an  active  manner, 
a  single  spermatozoan  penetrates  the  ovum,  this  accomplished,  the  head  and 
body  meet  and  unite  with  the  nucleus  of  the  ovum.  A  series  of  histologic 
changes  now  arise  which  eventuate  in  the  production  of  a  new  cell,  the  parent 


238  HUMAN  PHYSIOLOGY. 

cell,  from  which  the  new  being  develops  through  successive  division,  multi- 
plication and  differentiation  of  cells. 

The  Fixation  of  the  Ovum. — The  ovum  after  fertilization  in  the  oviduct, 
continues  to  divide  and  pass  slowly  to  the  uterus  (8-10  days)  where  it  is 
retained  until  the  end  of  gestation.  A  menstrual  mucosa  having  developed, 
the  ovum  lodges  on  a  smooth  thick  area  and  gradually  sinks  beneath  the  sur- 
face. During  the  passage  down  the  oviduct  the  zona  pellucida  has  become 
attenuated  and  has  been  finally  replaced  by  a  thick  layer  of  ameboid  and 
phagocytic  cells  called  the  trophoderm.  Upon  lodgment  of  the  ovum  these 
cells  destroy  the  underlying  mucosa  and  produce  a  cavity  into  which  the  ovum 
sinks.  As  the  ovum  increases  in  size  the  mucosa  gradually  covers  it;  that 
portion  of  the  mucosa  toward  the  uterine  cavity  is  called  the  decidua  capsu- 
laris  or  reflexa,  that  beneath  the  ovum  the  decidua  basilaris  or  placentalis, 
while  the  remainder  constitutes  the  decidua  parietalis  or  vera.  As  develop- 
ment proceeds  the  decidua  basilaris  becomes  greater  and  ultimately  develops 
into  the  placenta,  the  organ  of  nutrition  and  respiration. 

Segmentation  of  the  Ovum. — Immediately  after  fertilization  the  ovum 
divides  and  redivides  within  the  diminishing  zona  pellucida,  forming  an  irreg- 
ular mass  of  cells  called  the  morula.  The  peripheral  cells  form  a  layer,  the 
trophoderm  beneath  the  attenuated  zona  pellucida  ultimately  replacing  that 
structure.  The  remaining  cells  of  the  morula  differentiate  into  three  masses, 
ectodermal,  entodermal  and  mesodermal.  The  central  cells  of  these  masses 
liquefy  and  disappear  forming  thus  the  ectodermal  or  amniotic  cavity,  limited 
by  the  ectoderm;  the  entodermal  cavity  limited  by  the  entoderm;  and  the 
mesodermal  or  celomic  cavity  limited  by  the  extra  embryonic  mesoderm. 
Meanwhile  cells  in  various  parts  of  the  thickened  trophoderm  have  dis- 
appeared leaving  this  layer  in  the  form  of  delicate  trophodermal  villi,  the 
future  chorionic  and  placental  villi. 

The  Embryonic  Shield. — The  floor  of  the  amniotic  cavity  consisting  of 
ectoderm  and  entoderm  constitute  the  embryonic  shield  or  disk.  As  the 
shield  increases  in  size,  a  median  longitudinal  thickening  is  seen  occupying 
the  caudal  half  of  the  area.  This  is  the  primitive  streak,  a  temporary  struc- 
ture that  is  soon  overshadowed  by  changes  in  the  area  just  in  front  of  it. 
Here  is  formed  a  median  longitudinal,  grooved  ridge  of  ectoderm  that  devel- 
ops rapidly  in  length.  This  is  the  neural  groove  and  folds.  The  dorsal 
lips  of  the  groove  approach  each  other  in  the  mid-line  and  fuse,  separating 
from  the  original  ectoderm  which  closes  over  the  ectodermal  tube.  This 
tube  is  the  neural  tube  from  which  the  nerve  system  is  developed.  In  the 
immediate  vicinity  of  the  head  end  of  the  primitive  streak  is  seen  a  darkened 


REPRODUCTION.  239 

area,  Hensen's  node,  that  represents  the  beginning  invagination  of  the  ecto- 
derm in  the  formation  of  the  embryonic  mesoderm  and  notochord  to  be  con- 
sidered later.  That  portion  of  the  embryonic  shield  that  gives  rise  to  the 
embryo  itself  becomes  distinctly  outlined  laterally  and  in  the  head  and  tail 
regions  of  the  neural  groove.  Just  external  to  this  area,  the  embryonic  area 
proper,  is  a  transparent  area,  the  area  pellucida,  beyond  which  is  the  area 
opaca  in  which  the  first  blood-vessels  appear. 

Mesoderm  and  Notochord. — So  far  in  the  embryonic  area  only  ectoderm 
and  entoderm  exist.  Hensen's  node,  at  the  head  end  of  the  primitive  streak 
represents  an  invagination  (gastrulation)  of  ectoderm  between  ectoderm  and 
entoderm.  This  invagination  elongates  headward  in  the  embryonic  area 
constituting  a  tube  of  ectodermal  cells,  the  chordal  canal.  Later  the  ventral 
wall  of  the  canal  and  the  adjacent  entoderm  disappear,  so  that  the  chordal 
ectoderm  temporarily  forms  the  dorsal  median  boundary  of  the  entodermal 
cavity.  By  this  process  a  communication  is  established  between  the  ento- 
dermal cavity  and  neural  groove,  called  the  neuro-enteric  canal.  The  chordal 
ectoderm  separates  from  the  entoderm  and  then  forms  a  solid  cord  of  cells, 
the  notochord;  between  entoderm  and  neural  groove  the  neurenteric  canal, 
however,  persisting  for  some  time.  In  the  meanwhile,  other  ectodermic 
cells  in  the  region  of  the  chordal  invagination  spread  between  ectoderm  and 
entoderm  and  form  the  anlage  of  the  mesoderm.  These  cells  by  rapid  prolif- 
eration soon  separate  ectoderm  and  entoderm  and  join  the  extra-embryonic 
mesoderm.  The  separation  of  these  two  structures  is  complete  except  in 
the  regions  of  the  bucco-pharyngeal  and  cloacal  membranes. 

On  each  side  of  the  neural  groove  the  mesoderm  becomes  transversely 
grooved  in  its  ectodermal  surface  forming  a  number  of  successive  block-like 
masses  called  primitive  somites  or  segments;  of  these,  there  are  thirty-eight 
for  the  trunk  and  possibly  four  for  the  head  regions.  Each  segment  consists 
of  three  parts,  the  sclerotome,  the  myotome  and  the  dermatome.  Lateral 
to  the  somite  is  a  thickened  mass  of  mesoderm,  the  intermediate-cell  mass, 
that  laterally  divides  into  two  layers;  the  outer  accompanies  the  ectoderm 
forming  the  somatopleure,  which  gives  rise  to  the  body  wall;  the  inner  joins 
the  entoderm,  forming  the  splanchnopleure  from  which  the  gut  tract,  vitelline 
duct  and  yolksack  are  derived. 

Fetal  Membranes. — As  the  primitive  streak  and  neural  groove  are  forming, 
the  extra-embryonic  mesoderm  that  lies  beneath  the  trophoderm,  invades  the 
trophodermic  villi,  forming  there  the  chorion  with  its  villi.  Gradually  the 
mesoderm  of  the  roof  of  the  amniotic  cavity  divides  into  two  layers,  the  upper 
constituting  chorionic  mesoderm,  while  the  under  one  is  attached  to  the  ecto- 
derm of  the  amniotic,  and  forms  with  the  latter,  the  Amnion.     In  the  chick 


240  HUMAN  PHYSIOLOGY. 

and  some  mammals  the  amnion  is  derived  from  the  somatopleure  in  the 
folding  off  of  the  body.  In  amniotes  the  amniotic  cavity  is  at  first  small,  but 
rapidly  increases  in  size.  It  contains  a  clear  fluid,  the  amniotic  fluid,  which 
amounts  at  term  to  about  one  quart.  It  serves  to  protect  the  fetus  during 
gestation,  and  at  parturition  it  dilates  the  os  cervis  and  flushes  the  birth 
canal.  This  liquid  is  derived  mainly  from  the  blood  as  it  contains  albumin, 
sugar,  fat  and  inorganic  salts.  Traces  of  urea  indicate  that  some  of  its 
constituents  are  derived  from  the  embryo  itself. 

The  caudal  end  of  the  embryonic  area  is  left  connected  with  the  chorion  by 
a  heavy  band  of  mesoderm  termed  the  belly-stalk  to  which  the  caudal  part  of 
the  amnion  is  attached.  The  entoderm  is  invaginated  into  the  belly-stalk 
for  a  short  distance  constituting  the  allantois  of  higher  forms;  the  allantois 
grows  out  between  the  closing  somatopleure  folds  forming  the  body  wall 
and  constitutes  a  free  sack  upon  which  vessels,  allantoic  arteries  and  veins, 
develop  from  the  embryo.  This  sack  then  spreads  beneath  the  white  shell 
membrane  forming  the  organ  for  nutrition  and  respiration  of  these  forms 
during  the  last  half  of  their  incubation  periods.  In  mammals  the  extra- 
embryonic portion  of  the  allantois  is  of  little  importance. 

The  Formation  of  the  Placenta. — The  chorionic  villi  increase  rapidly 
in  size  and  number  and  usually  surround  the  whole  fetal  sack  giving  it  a 
peculiar  shaggy  appearance.  Blood  vessels  now  proceed  from  the  embryo 
along  the  belly-stalk  (not  the  allantois  in  higher  forms  as  formerly  stated). 
There  the  umbilical  arteries  and  veins  pass  to  the  chorionic  villi  and  send 
branches  to  those  of  the  placental  area;  these  vascularized  villi  constitute  the 
chorion  frondoswm,  while  the  avascular  villi  form  the  chorion  leva.  The 
villi  of  the  latter  disappear  during  the  second  month,  leaving  the  chorionic 
membrane  smooth.  The  villi  of  the  chorion  frondosum  now  penetrate  the 
uterine  glands  of  the  decidua  basilaris  which  by  this  time  have  been  denuded 
of  epithelium  and  have  gained  connection  with  the  blood-vessels  of  the 
mucosa;  in  this  manner  these  uterine  glands  have  become  converted  into 
blood  sinuses.  The  chorionic  villi  either  attach  themselves  to  the  tunica 
propria  of  the  mucosa  (fixed  villi)  or  remain  free,  floating  villi.  At  the 
edge  of  the  plancental  area  very  few  villi  develop  leaving  a  circular  channel 
called  the  marginal  sinus.  This  attachment  of  the  villi  becomes  marked 
from  the  third  month  on  and  is  considered  the  beginning  of  placentation. 
From  this  time  on  to  full  term  there  is  merely  an  increase  in  number 
of  villi  and  vessels  and  thus  an  increase  in  the  size  of  the  placenta. 

The  placenta  is  the  most  important  of  the  fetal  structures.  As  it  develops, 
conditions  are  established  which  permit  of  a  free  exchange  of  material  be- 
tween mother  and  child.     Whether  by  osmosis  or  by  an  act  of  secretion,  the 


REPRODUCTION.  241 

nutritive  materials  of  the  maternal  blood  pass  through  the  intervening 
membrane  into  the  fetal  blood  on  the  one  hand,  while  waste  products  pass 
in  the  reverse  direction  into  the  maternal  blood  on  the  other  hand.  Inas- 
much as  oxygen  is  absorbed  and  carbon  dioxid  exhaled  by  the  same  structures, 
the  placenta  is  to  be  regarded  as  both  a  digestive  and  a  respiratory  organ. 
So  long  as  these  exchanges  are  permitted  to  take  place  in  a  normal  manner 
the  nutrition  of  the  embryo  is  secured. 

The  Nutrition  of  the  Embryo. — As  the  ovum  passes  down  the  oviduct 
it  imbibes  nutritive  materials  from  the  mucosa.  As  it  lodges  in  the  uterus  it 
is  nourished  as  first  in  the  same  way.  The  first  circulation  developed  is  the 
vitelline,  but  as  the  amount  of  nutritive  material  is  very  small  in  mammals 
its  activity  is  limited.  In  the  oviparous  forms,  however,  where  the  nutritive 
material  is  large  in  amount  this  circulation  is  important.  The  allantoic  cir- 
culation is  likewise  of  importance  in  the  oviparous  forms  and  constitutes 
their  last  fetal  circulation.  In  mammals  the  allantoic  circulation  is  merely 
a  transitional  stage  in  the  formation  of  the  placental  circulation. 

Circulation  of  Blood  in  the  Fetus. — The  blood  returning  from  the 
placenta,  after  having  received  oxygen  and  being  freed  from  carbonic  acid, 
is  carried  by  the  umbilical  vein  to  the  under  surface  of  the  liver;  here  a  portion 
of  it,  about  one-half,  passes  through  the  ductus  venosus  into  the  ascending 
vena  cava,  while  the  remainder  flows  through  the  liver  and  passes  into  the 
inferior  vena  cava  by  the  hepatic  veins.  When  the  blood  is  emptied  into  the 
right  auricle,  it  is  directed  by  the  Eustachian  valve  through  the  foramen 
ovale,  into  the  left  auricle,  thence  into  the  left  ventricle,  and  so  into  the  aorta 
and  to  all  parts  of  the  system.  The  venous  blood  returning  from  the  head 
and  upper  extremities  is  emptied,  by  the  superior  vena  cava,  into  the  right 
auricle,  from  which  it  passes  into  the  right  ventricle,  and  thence  into  the 
pulmonary  artery.  Owing  to  the  condition  of  the  lung  only  a  small  portion 
flows  through  the  pulmonary  capillaries,  the  greater  part  passing  through 
the  ductus  arteriosus,  which  opens  into  the  aorta  at  a  point  below  the  origin  of 
the  carotid  and  subclavian  arteries.  The  mixed  blood  now  passes  down  the 
aorta  to  supply  the  lower  extremities,  but  a  portion  of  it  is  directed,  by  the 
hypogastric  arteries,  to  the  placenta,  to  be  again  oxygenated. 

At  birth,  the  placental  circulation  gives  way  to  the  circulation  of  the  adult. 
As  soon  as  the  child  begins  to  breathe,  the  lungs  expand,  blood  flows  freely 
through  the  pulmonary  capillaries,  and  the  ductus  arteriosus  begins  to  con- 
tract. The  foramen  ovale  closes  about  the  tenth  day.  The  umbilical  vein, 
the  ductus  venosus,  and  the  hypogastric  arteries  become  impervious  in 
several  days  as  far  as  the  bladder.  Their  distal  ends  ultimately  form 
rounded  cords. 
16 


242  HUMAN  PHYSIOLOGY. 

Physiologic  Activities  of  the  Embryo. — During  intrauterine  life  the 
evolution  of  structure  is  accompanied  by  an  evolution  of  function.  The 
relatively  simple  and  uniform  metabolism  of  the  undifferentiated  blastodermic 
membranes  gradually  increases  in  complexity  and  variety,  as  the  individual 
tissues  and  organs  make  their  appearance  and  assume  even  a  slight  degree  of 
functional  activity.  As  to  the  periods  at  which  different  organs  begin  to 
functionate,  but  little  is  positively  known. 

The  primitive  heart,  in  all  probability,  begins  to  pulsate  very  early,  as  in  an 
embryo  from  fifteen  to  eighteen  days  old  and  measuring  but  2.2  mm.  in  length, 
Coste  found  the  amnion,  the  allantois,  the  omphalo-mesenteric  vessels,  and 
the  two  primitive  aortae  developed.  In  the  earlier  weeks,  all  products  of 
metabolism  are  doubtless  eliminated  by  the  placental  structures;  but  as 
metabolism  increases  in  complexity  the  liver  and  kidney  assume  excretory 
activity.  Thus,  at  the  end  of  the  third  month  the  intestine  contains  a  dark, 
greenish,  viscid  material — meconium — composed  of  bile  pigments,  bile  salts, 
and  desquamated  epithelium;  the  amniotic  fluid,  as  well  as  the  fluid 
within  the  bladder,  contains  urea  at  the  end  of  the  sixth  month,  indicating  the 
establishment  of  both  hepatic  and  renal  activity.  Contractions  of  the  skeletal 
muscles  of  the  limbs  begin  about  the  fifth  month,  from  which  it  may  be 
inferred  that  the  mechanism  for  muscle  activity,  viz.,  muscles,  efferent  nerves, 
and  spinal  centers,  has  become  anatomically  developed  and  associated,  and 
capable  of  coordinate  activity.  These  contractions  are,  in  all  probability, 
automatic  or  autochthonic  in  character  due  to  stimuli  arising  within  the 
spinal  centers.     The  remaining  organs  remain  more  or  less  inactive. 

After  birth,  with  the  first  inspiration  and  introduction  of  food  into  the  ali- 
mentary canal,  the  physiologic  mechanisms  which  subserve  general  metab- 
olism begin  to  functionate  and  in  the  course  of  a  week  are  fully  established. 
At  this  time  the  cardiac  pulsation  averages  about  135  a  minute;  the  respira- 
tory movements  vary  from  30  to  35  a  minute,  and  are  diaphragmatic  in  type; 
the  urine,  which  was  at  first  scanty,  is  now  abundant  and  proportional  to  the 
food  consumed;  the  digestive  glands  are  elaborating  their  respective  enzymes, 
digestion  proceeding  as  in  the  adult.  The  hepatic  secretion  is  active  and  the 
lower  bowel  is  emptied  of  its  contents;  the  coordinate  activities  of  the  nerve-, 
muscle-,  and  gland-mechanisms  are  entirely  reflex  in  character.  Psychic 
activities  are  in  abeyance  by  reason  of  the  incomplete  development  of  the 
cerebral  mechanisms. 


REPRODUCTION. 


243 


TABLE  SHOWING  RELATION  OF  WEIGHTS  AND  MEASURES  OF 

THE  METRIC  SYSTEM  TO  APPROXIMATE  WEIGHTS  AND 

MEASURES  OF  THE  UNITED  STATES. 

MEASURES  OF  LENGTH. 


One  Myriameter 
One  Kilometer 
One  Hectometer 
One  Decameter 

One  Meter 

One  Decimeter 
One  Centimeter 


=  10,000  meters  =32,800        feet. 

=  1,000  meters  =   3,280        feet. 

=  100  meters  =      328        feet. 

=  10  meters  =        32.80  feet. 

[  the  ten-millionth  part  of  a  1 
=  <  quarter  of  the  Meridian  \  =39.368  inches. 


of  the  Earth 

the  tenth  part  of  1  meter 


•936 


inches. 


One  Millimeter       = 


One  Myriagram  = 

One  Kilogram  = 

One  Hectogram  = 

One  Decagram  = 

One  Gram  = 

One  Decigram  = 

One  Centigram  = 

One  Milligram  = 


One  Myrialiter       = 


One  Kiloliter 


One  Hectoliter        = 


f  the  one-hundredth  part  of  1  ,„.      .     , 

=   0.393  (!)     incb- 
{  one  meter  J 

the  one-thousanth  part  of 


one  meter 

WEIGHTS. 

10,000  grams 

1,000  grams 

100  grams 

10  grams 

the  weight  of  a  cubic  cen-  1 

timeter  of  water  at  40  C.    J 

the  tenth  part  of  a  gram 
the  1  ooth  part  of  1  gram 
the  thousanth  part  of  one  1 
gram  J 


0-039  (2V)    inch. 


=  26f  pounds  Troy. 
=  2f  pounds  Troy. 
=  Tii  ounces  Troy. 
=   2\  drams     Troy. 

=  15-434  grains. 

=    1-543  C1!)   grains. 
=   0.154  (i)     grain. 

=   0.015  (&)  grain. 


gallons. 


MEASURES  OF  CAPACITY. 

[  10   cubic   Meters  or   the  J 

measures  of  10  Milliers  of  [  =  2,600 
I  water 

1    cubic    Meter    or    the  ] 

measure  of   1   Millier  of  j-  =     260      gallons. 

water 

100  cubic  Decimeters  or 

the  measure  of  1  Quintal  [  =      26       gallons. 

of  water 


244 


HUMAN  PHYSIOLOGY. 


One  Decaliter 


One  Liter 


One  Deciliter 


One  Centiliter 


One  Milliliter 


10  cubic  Decimeters  or 
the  measure  of  i  Myria- 
gram  of  water 

i  cubic  Decimeter  or  the 
measure  of  i  Kilogram  of 
water 

f  ioo  cubic  Centimeters  or 
the  measure  of  i  Hecto- 
gram of  water 

10  cubic  Centimeters  or 
the  measure  of  i  Deca- 
gram of  water 

i  cubic  Centimeter  or  the 
measure  of  i  Gram  of 
water 


—        2.6  gallons. 


=        2.1  pints. 


=        3.3  ounces. 


=        2.7  drams. 


=      16.2  minims. 


INDEX. 


ABDUCENT  NERVE,  199 
Aberration,  chromatic,  221 

spheric,  221 
Absorption,  102 

by  lacteals,  106 

by  blood-vessels,  106 

of  oxygen  in  respiration,  106 
Accommodation  of  the  eye,  220 
Adipose  tissue,  uses  of,  in  the  body,  34 
Adrenal  bodies,  146 
Adult  circulation,  establishment  of,  at 

birth,  241 
Air,  atmospheric,  composition  of,  132 

amount  exchanged  in  respiration, 
132 

changes  in,  during  respiration,  133 
Alcohol,  action  of,  81 
Alimentary  principles,  classification  of, 
78 

carbohydrate  principles,  80 

protein  principles,  78 

oleaginous  principles,  79 

inorganic  principles,  79 
Amino-acids,  15 
Amnion,  formation  of,  268 
Animal  heat,  135 

Anterior  columns  of  spinal  cord,  166 
Aphasia,  190 
Area,  germinal,  239 
Areolar  tissue,  33 
Arteries,  properties  of,  123 
Articulations,  38 

classification  of,  40 
Asphyxia,  134 
Astigmatism,  221 

BILE,  100 
Bladder,  urinary,  151 
Blastodermic  membranes,  238 
Blood,  no 

composition  of,  plasma,  in 

coagulation  of,  114 

coloring-matter  of,  113 

changes  in,  during  respiration,  116 

circulation  of,  126 

rapidity  of  flow  in  arteries,  125 

rapidity  of  flow  in  capillaries,  126 

corpuscles,  112 

origin  of,  114 

pressure,  124 
Bone,  structure  of,  36 
Burdach,  column  of,  167 

CAPILLARY  BLOOD-VESSELS,   125 
Capsule,  internal,  180 
external,  180 


Carbohydrates,  8 
Cardiac  cycle,  119 
Cartilage,  35 
Caudate  nucleus,  180 
Cells,  structure  of,  26 

manifestations  of  life  by,  28 

of  anterior  horns  of  gray  matter, 
164 

reproduction  of,  30 
Center  for  articulate  language,  190 
Central  organs  of  the  nerve  system  and 

their  nerves,  162 
Cerebellum,  181 

forced  movements,  182 
Cerebrum,  183 

fissures  and  convolutions,  184 

functions  of,  186 

localization  of  functions,  188 

motor  area  of,  186 

sensor  centers  of,  192 
Chemic  composition  of  human  body,  7 
Chorda  tympani  nerve,  course  and  func- 
tion of,  202,  203 
Chorion,  240 
Chyle,  109 
Ciliary  muscle,  215 
Circulation  of  blood,  116 
Claustrum,  180 
Cochlea,  229 

Columns  of  spinal  cord,  166 
Connective  tissues,  physiologic  proper- 
ties of,  33 
Corium,  160 

Corpora  quadrigemina,  179 
Corpus  luteum,  235 

striatum,  180 
Corti,  organ  of,  229 
Cranial  nerves,  195 
Crura  cerebri,  178 
Crystalline  lens,  217 

DECIDUAL  MEMBRANE,  238 
Deglutition,  90 
Digestion,  86 
Ductus  arteriosus,  241 
venosus,  241 

EAR,  223 
Electrotonus,  76 
Embryo,  activities  of,  242 
Embryonic  shield,  238 
Endolymph,  230 
Epididymis,  236 
Eustachian  tube,  225,  228 
Excretion,  148 


245 


246 


INDEX. 


Eye,  212 

refracting  apparatus  of,  218 
blind  spot  of,  222 

FACIAL  NERVE,  201 

paralysis,  symptoms  of,  202 

Fallopian  tubes,  234 

Fat,  13 

Female  organs  of  generation,  233 

Fetus,  circulation  of  blood  in,  241 

Fissures  and  convolutions  of  brain,  184 

Foods  and  dietetics,  77 
animal,  84 
vegetable,  85 
cereal,  85 

percentage  composition  of,  85 
daily  amount  required,  82 
protein  principles  of,  79 
saccharine  principles  of,  80 
oleaginous  principles  of,  79 
inorganic  principles  of,  80 
energy  of,  82 

Fovea  centralis,  222 

GALVANIC     CURRENTS,     EFFECT 

on  nerves,  76 
Ganglia,  193 

ophthalmic,  193 

Gasserian,  193 

spheno-palatine,  193 

otic,  194 

sub-maxillary,  194 

semilunar,  194 
Gastric  digestion,  91 

juice,  91 

action  of,  95 
Generation,  male  organs  of,  236 

female  organs  of,  233 
Globules  of  the  blood,  112 

of  the  lymph,  106 
Glomeruli  of  the  kidneys,  150 
Glosso-pharyngeal  nerve,  204 
Glottis,  respiratory  movements  of,  130 
Glycogen,  158 

Glycogenic  function  of  the  liver,  158 
Goll,  column  of,  167 
Graafian  follicles,  233 

HAIR,  160 

Hearing,  sense  of,  223 

Heart,  116 

valves  of,  117 

sounds  of,  121 

influence   of   pneumogastric   nerve 
upon, 122 

ganglia  of,  122 

course  of  blood  through,  117 

influence    of    nerve    system   upon, 
122 
Hemianopsia,  197 
Hemoglobin,  113 
Hyaloid  membrane,  216 
Hypermetropia,  221 
Hypoglossal  nerve,  208 

INCUS  BONE,  224 

Inorganic  constituents  of  body,  22 

Insalivation,  87 

nerve  mechanism  of,  89 


Inspiration,   movements   of  thorax  in, 

129 
Internal  capsule,  180 

results  of  injury  to,  181 
Intestinal  juice,  98 
Internal  secretion,  142 
Iris,  214 

action  of,  214 

KIDNEYS,  148 

excretion  of  urine  by,  155 

LABYRINTH    OF  INTERNAL  EAR, 
228 

function  of  cochlea,  230 

function  of  semicircular  canals,  230 
Language,  articulate,  center  for,  190 
Larynx,  231 

Lateral  columns  of  spinal  cord,  166 
Laws  of  muscular  contraction,  76 
Lens,  crystalline,  217 
Levers,  55 
Lime  phosphate,  22 
Liver,  155 

secretion  of  bile  by,  157 

production  of,  glycogen,  158 

formation  of  urea,  159 
Localization  of  functions  in  cerebrum, 

188 
Lungs,  128 

changes    in    blood    while    passing 
through,  116 
Lymph,  108 
Lymphatic  glands,  104 

vessels,  origin  and  course  of,  104 

MALLEUS  BONE,  224 
Mammary  glands,  140 
Mastication,  86 

nerve  mechanism  of,  87 

muscles  of,  86 
Medulla  oblongata,  174 

properties  and  functions  of,  176 
Membrana  tympani,  224 
Menstruation,  235 
Mesoderm  and  notocord,  239 
Middle  ear,  223 
Milk,  141 

Motor  centers  of  cerebrum,  186 
Muscles,  properties  of,  42 

changes  in,  during  contraction,  40 

special  physiology  of,  54 
Muscle-fiber,  histology  of,  44 
Myopia,  221 

NERVE,  OLFACTORY,  196 
optic,  197 
motor  occuli,  198 
trochlearis,  199 
trigeminal,  199 
abducent,  199 
facial,  201 
acoustic,  203 
glosso-pharyngeal,  204 
pneumogastric,  205 
spinal  accessory,  207 
hypoglossal,  208 
cells,  structure  of,  61 


INDEX. 


247 


Nerve,  fibers,  structure  of,  63 
terminations  of,  67 

impulse,  rate  of  transmission  of,  75 

roots,     function    of    anterior    and 
posterior,  68 

tissue,  histology  of,  60 

trunks,  structure  of,  64 
Nerves,  afferent,  86 

efferent,  69 

classification  of,  66 

degeneration  of,  70 

relation  of,  to  spinal  cord,  68 

development  and  nutrition  of,  68 

cranial,  145 

vaso-motor,  177 

properties  and  functions  of,  73 

spinal,  68,  167 
Nerve  tissue,  physiology  of,  60 

sympathetic,  193 
Neuron,  60 
Nucleus  caudatus,  180 

lenticularis,  180 

OLFACTORY*;  NERVES,  196 
Ophthalmic  ganglion,  193 
Optic  nerves,  197 

thalamus,  180 

functions  of,  180 
Organs  of  corti,  229 
Osazones,  12 
Otic  ganglion,  194 
Ovaries,  233 
Ovum,  233 

discharge  of,  from  the  ovary,  234 
Oxygen,  absorption  of,  by  hemoglobin 
113 

PANCREATIC  JUICE,  99 

Parathyroids,  145 

Peptones,  95 

Perilymph,  229 

Perspiration,  162 

Petrosal  nerves,  large  and  small,  203 

Phonation,  232 

Physiology,  definition  of,  32 

Placenta,  formation  and  function  of,  240 

Pleura,  128 

Pneumogastric  nerve,  205 

Pons  variolii,  178 

Portal  vein,  106 

Posterior  columns  of  spinal  cord,  167 

Prehension,  86 

Presbyopia,  221 

Pressure  of  blood  in  arteries,  138 

Proteins,  14 

classification  of,  17 

use  of,  in  the  body,  79 
Proximate  principles,  80 

inorganic,  22 

carbohydrates,  8 

fat,  13 

proteins,  14 

of  dissimilation,  25 
Ptyalin,  89 
Pulse,  125 
Pyramidal  tracts,  166 

RED   CORPUSCLES  OP  BLOOD,  112 


Reflex  movements  of  spinal  cord,  168 

action,  laws  of,  71 
Reproduction,  233 
Respiration,  127 

movements  of,  129 

types  of,  131 

nerve  mechanism,  130 
Retina,  130 
Rigor  mortis,  46 

SALIVA,  88 
Sebaceous  glands,  161 
Secretion,  137 
Semen,  237 

Semicircular  canals,  228 
Sight,  sense  of,  212 
Skeleton,  physiology  of,  38 
Skin,  160 

Smell,  sense  of,  212 
Sounds  of  heart,  121 
Spermatozoa,  237 
Spheno-palatine  ganglion,  193 
Spinal  accessory  nerve,  207 

cord,  164 

cord,  membranes  of,  163 

structure  of  white  matter,  166 

structure  of  gray  matter,  164 

properties  of,  173 

function  of,  as  a  conductor,  171 

as  an  independent  center,  167 

reflex  action  of,  168 

special  centers  of,  170 
reflex  movements,  169 

paralysis  from  injuries  of,  207 

nerves,  origin  of,  196 
Starvation,  phenomena  of,  77 
Stomach,  91 

Submaxillary  ganglion,  194 
Sudoriparous  glands,  161 
Sugar,  uses  of,  in  the  body,  80 
Supra-renal  capsules,  146 
Sympathetic  nervous  system,  193 

properties  and  functions  of,  194 

action  on  heart,  123 

TASTE,  SENSE  OF,  210 

nerve  of,  211 
Teeth,  86 

Tensor  tympani  muscle,  224,  229 
Testicles,  236 
Thoracic  duct,  106 
Thorax,  enlargement  of,  in  inspiration, 

129 
Tissues,  histology  of,  31 
Thyroid  gland,  143 
Tongue,  210 

motor  nerve  of,  211 

sensory  nerve  of,  211 
Touch,  sense  of,  209 
Trochlearis  nerve,  199 
Ttirck,  column  of,  166 

UREA,  153 

Uric  acid,  154 

Urination,  nerve  mechanism  of,  151 

Urine,  152 

composition  of,  153 

average    quantity    of    constituents 
secreted  daily,  153 


248  INDEX. 

Uterus,  234  Vesiculae  seminales,  237 

Villi,  structure  and  functions,  107 

VAGUS  NERVE,  205  Vision,  psychic  center  for,  192 

Vapor,  watery,  of  breath,  133  Vital  capacity  of  lungs,  132 

Vascular  glands,  102  Vocal  bands,  231 

Vaso-motor  nerves,  origin  of,  177  Voice,  231 
Veins,  126 

Vertebral  column,  42  WATER,  AMOUNT  OF,  IN  BODY,  22 


FOR      MEDICAL      STUDENTS 

WEBSTER'S    DIAGNOSTIC 

Mr>qr  ij  r^  f\  Q     chemical,  bacteriological, 
bl    nUUO       AND       MICROSCOPICAL 

By  Ralph  W.  Webster,  m.d.,  ph.d.,  Asst.  /'  of Pharma 

Therapeutics,  and  Instructor  in  Medicine^  Rush  M<  ./'/■  al  College  I  rnivt  r  ity  of 
Chicago}\  Pathologic  Chemist,  Cook  County  Hospital.  Third  Edition, 
Revised,  Enlarged,  xxxvii  4-  692  pages,  with  37  Colored  Plates  and  [I  | 
other  Illustrations.  Cloth,  #4.50 


MONTGOMERY'S    PRAC- 
TICAL     GYNECOLOGY 

By  Edward  E.  Montgomery,  m.d.,  Professor  of  Gynecology  in  Jefferson 
Medical  College,  Philadelphia ;  Gynecologist  to  the  Jefferson  and  St.  Joseph' 's 
Hospitals,  etc.  Fourth  Edition.  Rearranged.  Thoroughly  revised  and  in 
part  rewritten.  With  589  Illustrations,  many  of  which  are  new.  3  in  colors. 
Octavo.     879  pages.  Cloth,  $6.00 


EDGAR'S     OBSTETRICS 

FOURTH    EDITION 

By  J.  Clifton  Edgar,  m.d.,  Professor  of  Obstetrics  and  Clinical  Mid- 
ivifery,  Medical  Department  of  Cornell  University,  New  York  City;  Attending 
Obstetrician  to  the  New  York  Maternity  Hospital,  etc.  Fourth  Edition,  Re- 
written and  Revised.  13 1 6  Illustrations.  38  Figures  in  Colors.  Svo.  1050 
pages.  Cloth,  $6.00 


BINNIE'S    OPERATIVE 
SURGERY 

By  JOHN  FAIRBAIRN  IilNNIE,  A.M.,  CM.  (Aberdeen);  Surgeon  to  the 
General  Hospital,  Kansas  City,  Missouri;  Fell 010  of  the  American  Su 
Association ;  Membre  de  la  Societe  Internationale  de  Chirurgie,  etc.  Fifth 
Edition  in  one  octavo  volume.  Thoroughly  revised  and  enlarged.  1365 
Illustrations,  some  of  which  are  printed  in  colors,  over  100  being  new. 
x  -f-  1 153  pages.  Cloth,  $7.00 


P.  BLAKISTON'S    SON    &   CO..   Publishers 
PHILADELPHIA 


The  Practice  of  Medicine 


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